Honda-SOHC

SOHC.co.uk Forums => Other Bikes => Topic started by: Laverda Dave on January 23, 2017, 11:18:55 PM

Title: CB250RS Rebuild Progress
Post by: Laverda Dave on January 23, 2017, 11:18:55 PM
Hi All.
Thought I'd share the progress of the CB250RS rebuild with you all.
The idea is to restore the bike without spending an absolute fortune as lets face it, they are not worth a lot which is a shame as they are lovely little bikes. I thought I'd restore this one as it has served me well for 12 years and its about time the more obscure SOHC CB's were restored otherwise there wont be any left in a few years.
I began the strip down over Christmas with the intention to strip, label and photograph everything as it came apart.  I have decided that unless the cylinder head is in good condition (or I can get decent replacements if it isn't) it won't be worth restoring.
I've attached a couple of photos of progress so far, the bike is now all but striped into chassis component parts and the engine as a complete unit. 
Today I began stripping the engine, the moment of truth!
Initially all looked good, the cam head journals are in near perfect condition.......unfortunately the cam lobes are marked and pitted with the corresponding pad of the rocker arms also being scored.
Does anyone know if these can be reclaimed, years ago I had a can and followers metal sprayed and re-profiled but I don't know anywhere that does this service now, any recommendations would be helpful :)  DK have one on e-bay but it appears to be worse then mine! Unless I can find somewhere to reclaim these the engine is toast :(
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on January 24, 2017, 06:50:22 AM
Have you tried dave silver for parts, some obscure stuff can be stupidly cheap
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on January 24, 2017, 02:22:51 PM
DS advise cams are no longer available. CSML advise the same.
I'm going to send it to Newman Cams, £45 for a regrind that they say is possible as these cams apparently are child iron and the hardening is quite deep. They will resurface the pads on the rocker arms @£37 each. This is not going to be an 'inexpensive' restoration!
Title: Re: CB250RS Rebuild Progress
Post by: kettle738 on January 26, 2017, 05:21:23 PM

I would have suggested Newman Cams if I'd seen this earlier, having used them several times......their work is very specialised and in my experience has always been top notch.....so it bodes well for your cylinder head.

Mick.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on January 26, 2017, 09:22:53 PM
I had a spot off luck with the cam, someone on ebay was selling a brand new one still in its packet. I offered £50 and it was accepted.  I'll send the rockers to Newman Cams though as they need some help. They have a good reputation and do a lot of work for F1 teams. Mike Newman actually dealt with my call and was very helpful with advice.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on January 29, 2017, 05:14:11 PM
Some post-weekend progress photos following a good Saturday :) but a frustrating Sunday  :( in the garage.
The engine is now apart in large lumps with some bits going into the parts washer for an initial clean.  The crankcase is still together, I need to ask Mr Honda dealer up the road if I can borrow his 22mm x 1.6mm pitch puller, if not I'm going to have to buy one to use it once!
A photo of the camshaft is attached, I've tried to show the pits, the photo doesn't do it justice, the pits are quite deep. I've managed to obtain a NOS part on e-bay :D. The rocker pads are both pitted, although difficult to see in the photo. I'll be sending these to Newman Cams for some specialist refacing treatment.
The most frustrating problem is the pins holding the rocker spindles into the cover, the Haynes manual says apply some heat to get them out, I did that, they still didn't budge, alternatively the Haynes manual advises to use a dremel to grind a slot into the pins and lever them out, I did that and they still won't budge  >:( I've now soaked them in duck oil.  Any ideas anyone, they are too small and close to the casting to get mole grips on, I might have to get them spark erroded out, swearing at them hasn't worked either??????
Dave
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on January 30, 2017, 07:25:29 AM
Needle nose mole grips(yes they exist) along with heat mate
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 26, 2017, 10:50:37 AM
I've admitted defeat.....I cannot remove the two hardened pins holding the rockers into the head. I've tried everything,  penetrating oil, heat (lots of it) and swearing  >:(. The result is one pin snapping flush with the surrounding rocker box casting. I attempted to drill it out but the material is smoothed hard it blunted the centre punch!
Last resort now will be to spark erode the remaining stud out.
Does anyone know of an engineering company in West London or Middlesex that has this equipment?  I've tried a few places without success.
Title: Re: CB250RS Rebuild Progress
Post by: Tomb on February 26, 2017, 02:30:21 PM
Try a masonry drill sharpened for steel drilling, keep the tip cool/wet as the heat generated will melt the braze and the tungsten carbide tip will come out (DAMHIK :o)

Sometimes a good quality high chrome steel will work harden when drilling the surface, even just a resharpened HSS drill bit will cut through this with slow speed and cutting compound, compound for tapping holes works well.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 26, 2017, 11:25:03 PM
Thanks for the advice Tomb, I'll give it a go if what a mate told me turns out to be correct, he was charged £80 a few years ago to have a 6mm exhaust stud spark eroded out of a cylinder head. If true I'll be looking at the thick end of £200 to remove these studs  :'(.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on April 22, 2017, 02:46:30 PM
The bike is now in 100's of bits  :o.  Just about to load it all up and take it to the aqua blaster at I Cleenz Machineez in Penge a quote.  I'm going with stove enameling this time for the frame and other painted parts.  All the nuts and bolts have been cleaned up and the burrs taken off before dropping off at the chrome/zinc platter. I have filled the rough passivated casting posing as a kick start lever, I have polished it and will have this chromed along with the rear brake pedal, I know its not stock but a grey rough casting really ruins the look of the black engine. ;)
Piki, do you have the DID decals for the wheel rims in stock and the tank, mudguard and seat decals?  I'm already thinking about the finishing touches  ::)
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on June 12, 2017, 07:56:53 PM
Chrome Porn  :) I collected this little lot from the chromer today, fantastic job he made of everything.  £98 all in. Before anyone says anything.... I choose to plate the kick-start and rear brake lever, I couldn't stand the rough finish of the originals. I took me a full day to file the kick-start shaft and knuckle smooth and blend the contours.
Title: Re: CB250RS Rebuild Progress
Post by: Green1 on June 12, 2017, 08:02:24 PM
Nothing wrong with a bit of bling
Chrome looks good on almost everything 
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on September 23, 2017, 11:29:57 AM
A bit of progress to report.
The rocker arms have been refaced and ground by Newman Cams and arrived back today, lovely job  :).
Piki has manufacture of the new decals in progress and I'm cleaning up the panels to send to Menno for paint.
I'll post some more photos later of progress so far. The other half passed her module 1 bike test 10 days ago on the same day we went on holiday, module 2 is about to be booked so the pressure is now on to rebuild the bike for her to use!
Title: Re: CB250RS Rebuild Progress
Post by: JamesH on September 23, 2017, 12:18:16 PM
Chrome Porn  :) I collected this little lot from the chromer today, fantastic job he made of everything.  £98 all in. Before anyone says anything.... I choose to plate the kick-start and rear brake lever, I couldn't stand the rough finish of the originals. I took me a full day to file the kick-start shaft and knuckle smooth and blend the contours.
Who did the chrome? Looks very nice...
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on September 23, 2017, 02:44:00 PM
Newman Cams reprofiled the rockers, £37.50 each. Not cheap but you get what you pay for and they were really helpful.  Their phone number is 01689 857109.
They told me they have a new cam for a 400/4!
Title: Re: CB250RS Rebuild Progress
Post by: JamesH on September 23, 2017, 02:45:13 PM
Sorry, I meant who did the chrome plating. Looks lovely..!
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on September 23, 2017, 02:54:47 PM
Sorry James, I read your post too quickly. Chrome and zinc plate was done by Gary at Hayes Electrofinishing in Middlesex.  He's great, does a fantastic job, loves old Jag bikes and V Dub campers. He only does work for people he knows well, I've known him nearly 30 years. If you need anything doing let me know and I'll ask him. You'll need to drop it off and collect it though, he doesn't like doing all that as he's really geared up for industrial work but likes doing some bike stuff for a bit of variety. Every time I go there its touch and go if he'll be staying due to the environmental issues of his work.
Title: Re: CB250RS Rebuild Progress
Post by: JamesH on September 23, 2017, 04:01:12 PM
Top man. I'll drop you a pm..James
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 12, 2017, 05:59:48 PM
Some good progress to report this weekend on the rebuild. Although I seem to have spent the previous two days over the parts washer! I stripped the gear clusters and gear change mechanism down to their component parts and gave everything a good clean. The gearbox bearings, dogs, shafts and gears are all in perfect condition having used a strong magnifying glass to check all the teeth. The clusters are now all back together oiled, bagged and put to one side ready for engine assembly.
I've also given the paintwork a good scrub prior to packing and sending to Menno tomorrow in a huge box (it weighs 11.5kg fully packed!!!). I'm going to change the colour to red as it was the optional colour at the time.
Cylinder head assembly tomorrow  :)
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on November 29, 2017, 03:21:34 PM
Hi,
I'm new on here and stumbled accross this thread by googling 'removing cb250rs cylinder head cover pins'  ;D
I wonder how you eventually got those out ?
I'm at the stage where I have dremmelled a groove in them but still nothing is budging. Was going to attempt to Tig something on them this coming weekend, just a bit wary of melting the bloody thing.
Really interesting thread (for me at least) as I'm in the process of renovating an RS myself and have bits all over the place right now.
The chroming you've had done looks great. I got several quotes for some of the same parts (without gear change, brake pedal and kickstart) and prices were just too much, double what you've paid. Going to try a bit of DIY nickel and see how that turns out.
Anyway, will be following with interest.

Regards,
Huw


Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on November 29, 2017, 03:48:37 PM
Have a NOS rocker for one of these on eBay at the mo' ..if anyone is interested in it make me a reasonable offer.

https://www.ebay.co.uk/itm/142600335322?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
Title: Re: CB250RS Rebuild Progress
Post by: hairygit on November 29, 2017, 03:49:13 PM
Hi, and welcome to the forum. 250 RS was a great little bike, and for whatever reason never sold as well as the Superdream 250. Much nicer bike than the Superdream though, and would leave a Superdream for dead on acceleration and handling, much lighter and more "flickable" too.

Sent from my X6pro using Tapatalk

Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 30, 2017, 07:27:53 AM
Welcome to the forum Huw. There's great people on here with lots of advice.
The pins, yes the dreaded never to be removed pins  >:(! In the end nothing worked to get them out, I cut a groove in one of them as you have, it set up a weak point and it snapped! I got them both spark eroded out in the end, £50 but that was in the form of a 'drink' for the guy who did the work in his lunch breaks. The company I took it to work on aero engines and when they saw it quoted silly money, they didn't want to do it. In the end a guy who is into bikes said he'd give it a go in his lunch breaks. Spark eroding is the only way to get them out, they really are hard and brittle.
I'll be starting engine assembly over the weekend and will post some photos of progress.
Good luck with your rebuild.
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on November 30, 2017, 08:36:22 AM
Following this post with interest. I am pretty sure one of the magazines did a CB250RS rebuild a few years back (possibly CMM in Rod Gibson's time).

Interested in the cam/rocker refurb , as I have a couple of Bomber cams that have perfect journals but one lobe is pitted. Any prices to share on camshaft work by Newman Cams?

Looking at my NOS rocker the pad profile looks to have more curvature on the edges than the reworked ones. Probably of no significance as Newman always come up as being the 'go-to' place for camshaft work. Do they Stellite (or similar) weld and then grind the profile? Will be a lovely example when finished.

Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 30, 2017, 07:49:31 PM
More photos to follow over the weekend Ash. I'd be interested in seeing that rebuild article and see if they had the fun I had (and now Huw) getting those pins out  >:(!
Newman Cams did a great job with the rocker arms but unfortunately....... they did grind the sides of the pads down to the correct shape. It was only after I put them back in the cover with the hard to fit (and extract) and tried the cam I realised the exhaust cam lobe also opened the inlet rocker at the same time  :o. To be fair to Newman's there was no quibble at all. They asked me to send it back, paid the postage and sorted it out and sent them back within 24 Hours, great service and I wouldn't hesitate to use them again. I haven't looked at your listing on ebay but how much do you want for your rocker arm, it's good to have a spare?
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on November 30, 2017, 09:34:01 PM
Deal done with Dave on the NOS  rocker  :)
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on December 01, 2017, 09:37:42 AM
I think the rebuild was in CMM and possibly the last resto that Rod Gibson did before he sadly passed away .... I may have a couple of copies with part of the rebuild in Dave. I will look over the w/end.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 01, 2017, 06:46:50 PM
Thanks Ash. PM me your details for the rocker arm.
I'm just back from dropping the rebuilt wheels off at the tyre shop, collect tomorrow after visiting Kempton Park to collect the TEC shocks.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 02, 2017, 02:59:58 PM
Just back from Kempton Park autojumble with a new pair of TEC shocks. Stopped off at the tyre fitter on the way back to collect the wheels with new Roadriders fitted. Wheels are now all complete with stainless spokes, repainted hubs, new bearings and vapour blasted rims.
Engine rebuild to begin tomorrow.
Title: Re: CB250RS Rebuild Progress
Post by: MarkCR750 on December 02, 2017, 07:26:27 PM
Look very nice, have fun getting the tyre fitting “soap” off!
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on December 03, 2017, 01:56:50 AM
I think the rebuild was in CMM and possibly the last resto that Rod Gibson did before he sadly passed away .... I may have a couple of copies with part of the rebuild in Dave. I will look over the w/end.

I don't have that engine strip copy  to scan Dave but you can download it here...think this is bottom end though

https://magazinesubscriptionsdigital.zinio.com/browse/issues/index.jsp;jsessionid=ABDC36CB6B47EECEEDE18446756B7E99.prd-main-news3?skuId=416161189&prnt=&categoryId=cat1960016
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on December 03, 2017, 11:53:47 AM
Just found the CMM issue you need Dave I will scan it and post ...Thesame issue also had a very mistake ridden short section on my 400/4 by Mark williams of BIKE fame !
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on December 03, 2017, 05:41:15 PM
Here you go Dave ..I found both articles on the engine strip,scanned & PDF'd  :). Give it a short time though as it's a big file and taking a while to upload.

https://www.dropbox.com/s/318cabahyog0q9g/CB250RS_Strip_CMM_2011.pdf?dl=0
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 03, 2017, 09:04:28 PM
Wow, fantastic, thanks a million Ash. I was going to start the reassembly today but been too busy! I'll be out in the garage tomorrow on my day off and after reading these articles first :).
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on December 04, 2017, 07:27:30 PM
That's a good article. Didn't appear to have any particular problems with those rocker arm pins.
Here's the status of mine.
After dremmelling a grove[attach=1]
After tig attachment has broken off [attach=2]
After one last desperate wrench with the mole grips. [attach=3].

Last chance saloon with the drill next.
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on December 04, 2017, 07:47:02 PM
I'm halfway through my wheels. Rear has been respoked (stainless) and the front will be going to hagons within the next week. It's not a nuts and bolts restoration so I've made my wheels nice and shiney.  8)
[attach=1]

I did have roadriders fitted but have dumped these, mainly because they were oversize. My own fault. I've got some soft heidenau's to go on.

Also I have fitted a twin pot caliper and larger disc, which possibly came from a 750 but could've been from pretty much anything.

[attach=2]

Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 04, 2017, 08:05:16 PM
Looking good Huw. Those pins are a pain especially when you reach the point of no return and they have to come out.
I think you may go the route I took with burning them out.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 04, 2017, 09:30:26 PM
Some good progress today, I began the engine rebuild this afternoon.  All going well so far with no mishaps. I bought a new balance chain after checking the old one and found it had stretched by over 20% despite only having 14k on the engine. The  original Honda chains (DID) are no longer obtainable but having spoken to Andy the chain man he measured it and advised it's actually a Go-kart drive chain, £12 for a racing spec DID chain compared to £280 for a genuine poorer quality item on e-bay!
Next job is to join the cases and fit the ancillaries before moving on to the top end next weekend.
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on December 04, 2017, 09:34:59 PM
My  wife posted your  NOS rocker today Dave  :)
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 04, 2017, 10:10:20 PM
Thanks Ash. I bet it arrives quicker then the parts I ordered from CSMNL over a week ago and I'm still waiting for!
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 04, 2017, 10:15:02 PM
Huw, I've just looked in close up at your rocker pads, they look OK, do you need to take them out?
I bought some special 500c heat proof adhesive at Kempton on Saturday, I'll need to glue my new pins in as although they will still be a tight fit they won't be a tight as before the old ones were spark eroded out.
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on December 05, 2017, 03:39:47 PM
No I probably didn't or don't need to take them out. Just that while I'm cleaning/painting everything it seemed worthwhile doing that. Would rather replace them now.  I certainly would've gone for a new balancer chain if I'd realised there were ready made replacements available, especially at that price. I've got bottom end pretty much together now though and reluctant to split cases again for this.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on December 05, 2017, 03:48:15 PM
Thanks Ash. I bet it arrives quicker then the parts I ordered from CSMNL over a week ago and I'm still waiting for!
I put in a big parts order with CMSNL on 11th October. All items were in stock but then they discovered 1 of the items wasn't in stock ( computer error they say !!!). Still waiting, estimated dispatch 12th December but, not guaranteed yet......2 bloody months!!!
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on December 31, 2017, 03:37:37 PM
Whilst I await the removal of the locking dowel from the rocker cover to enable me to complete the engine reassembly I turned my attention to rebuilding the carb. At least this is one completed job. I'll start on reassembly of the frame and forks tomorrow.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on January 08, 2018, 08:01:09 PM
Getting back into the rebuild now after Christmas.
Front forks fully assembled. I had the stanchions ground and chromed by Philpotts, repainted fork bottoms and rechromed top nuts.
I also took the front brake apart and had this blasted as it was in a pretty sorry state. Just ordered some PJI gloss black engine enamel ready for a repaint.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 12, 2018, 08:04:36 PM
First day of my week off and spending it on a self catering holiday in the garage to rebuild the CB250RSA  :).
A very good day today, I have a rolling chassis with rebuilt wheels, repainted frame, reground forks, new seals, replated nuts, bolts and spindles.
The new rear shocks don't actually fit, the upper bush is too small so the old shocks are fitted for now.
Piki's decals should be here soon along with the paintwork from Menno.
Title: Re: CB250RS Rebuild Progress
Post by: haynes66 on February 12, 2018, 09:37:17 PM
this is my favourite stage of a rebuild.  it's coming on well now. i had an RS many years ago, great little bike.
Title: Re: CB250RS Rebuild Progress
Post by: Piki on February 13, 2018, 08:39:21 AM
Looks great!! Very good Job!
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 14, 2018, 07:24:18 PM
Rebuilt engine and carb has gone in the frame today.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on February 14, 2018, 07:34:46 PM
Progress indeed. It's looking good.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 20, 2018, 10:02:05 PM
Some more progress over the past few days. Rear shocks now fitted (Tec's), front brake fully overhauled with new stainless piston, black coated stainless hose and new master cylinder (old one was leaking badly and pretty rough). Carb ultrasonically cleaned and rebuilt, all new cables. Just begun installing the original loom, lucky enough this was in great condition and came up like new after 65c dishwasher cycle! 
The black plastic trim and rubber parts came up a treat after a good scrub and few days soaking in auto glym rubber restorer, thanks for the restoration tips everyone.
Hopefully it will be ready for a trial start up over the weekend.
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on February 21, 2018, 05:42:56 PM
Slightly off topic but I remember an insurance job where some scrote undid the headlamp bolts on an RS and the just cut every wire going into the shell, damn near wrote the bike off and never could get some of the special washers/collars---had to adapt what we had in stock but owner was just glad to get it back
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 22, 2018, 07:49:07 AM
I think it was one of your previous 'top tips' about placing the loom in the dishwasher. Came up like brand new :).
The same tip also works for the rubber parts followed by a good soaking in rubber restorer for a few days.
I did have to run the dishwasher empty afterwards with a dishwasher cleaner to remove evidence I had been using the kitchen appliances for motorcycle related activities  ::)
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on March 05, 2018, 07:44:15 PM
Good progress today on my day off. All the wiring is complete and surprisingly everything worked and I had a big fat spark. The front and rear brakes are now all set and adjusted with the biggest job, fitting the pattern exhaust also giving surprisingly few problems.
I've also added some non-standard chrome bling with the kick-start and rear brake levers.
All that is left to do now is fit the paintwork and run it  :)
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on March 05, 2018, 08:13:55 PM
Looks really good,  I've always liked the style of those and nicely offset with the spoked alloy rims. A good looking bike.

Quite a few of them used to hoon around in London on courier work if I remember correctly. Seem quite tough little engines.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on March 05, 2018, 09:58:19 PM
That does look blingy, very nice. What colour are you having the tank etc?
Title: Re: CB250RS Rebuild Progress
Post by: Spitfire on March 06, 2018, 10:17:18 AM
That looks ever so good, makes me want to get out into the garage and get stuck into the F2.

Cheers

Dennis
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on March 06, 2018, 01:11:45 PM
I'm sticking with the original blue Julie. I was going to change it to red but I've had enough red bikes!
Menno had a few problems obtaining the exact match but these are now overcome and he's in the process of completing the repaint. Piki has made the decals and sent these on to Menno.
I can't wait to see the finished result and neither can the O/H as it's going to be her bike :). Hopefully we'll get some miles in over easter.
Thanks for the nice comments Spitfire, the snow's gone and the temperature is nearly tropical compared to last week so your garage is calling for you  ;).
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on March 06, 2018, 02:07:21 PM
I'm sticking with the original blue Julie. I was going to change it to red but I've had enough red bikes!
Menno had a few problems obtaining the exact match but these are now overcome and he's in the process of completing the repaint. Piki has made the decals and sent these on to Menno.
I can't wait to see the finished result and neither can the O/H as it's going to be her bike :). Hopefully we'll get some miles in over easter.
Thanks for the nice comments Spitfire, the snow's gone and the temperature is nearly tropical compared to last week so your garage is calling for you  ;).

Hi,
Is it someone on here that has made your logos ? I can see a couple of firms on ebay do them. Do you know anything about these or the quality. I will be redoing mine in the original cosmo balck.  One problem I have is that the blue on the middle of the tank is not listed by honda. Only thing I can think of is to buy a logo set and then get those matched to paint. Or even paint all the blue bits.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on March 06, 2018, 05:32:12 PM
I have used Piki, he's a forum member on here. Piki made the decals for my 400/4 and did an excellent job. I've seen the kits on ebay, some good and bad reviews though. For me there's only one person and that's Piki.
The centre section of the petrol tank is painted but the two thin white lines either side are decals. The centre section of the rear seat cowl is also a full decal.
Post some photos of your rebuild, we all like a restoration  :)
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on March 07, 2018, 07:33:54 AM
I have used Piki, he's a forum member on here. Piki made the decals for my 400/4 and did an excellent job. I've seen the kits on ebay, some good and bad reviews though. For me there's only one person and that's Piki.
The centre section of the petrol tank is painted but the two thin white lines either side are decals. The centre section of the rear seat cowl is also a full decal.
Post some photos of your rebuild, we all like a restoration  :)
Yes my point about the central tank part is that Honda do not list the colour so it seems to me that the only way of getting this right is to get paint matched to the tail piece decal. Is that what you have done ?
I've not been too active recently but hoping to put engine in frame this weekend. So maybe photos then. It doesn't help that the frame is in a different location to where I live. Still haven't sorted the cam pin problem though.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on March 08, 2018, 07:56:45 AM
The paintwork is with Menno and he's added the centre colour on the tank to match the rear decal supplied by Piki.
I think you are going to have to bite the bullet and get those pins spark eroded out, there is no other way and I tried everything! You'll need to supply a new pin to the spark eroder as they will need to determine the depth to erode to. These ate obtainable from Honda.There are three of these pins in the head, two in the rocker arms and one locating the valve lifter. All three pins are the same length and diameter. The pin holding the valve lifter fell out,  shame the same didn't happen with those holding the rockers  :'(.
Good luck and keep posting photos.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on March 29, 2018, 07:52:46 AM
Menno has completed the paintwork, can't wait for its return and fitting. Fantastic paint by Menno with decals by Piki.
Title: Re: CB250RS Rebuild Progress
Post by: Piki on March 30, 2018, 08:11:39 PM
Woooov looks really fantastic!!!
Title: Re: CB250RS Rebuild Progress
Post by: Menno on April 03, 2018, 09:42:28 AM

[/quote]
Yes my point about the central tank part is that Honda do not list the colour so it seems to me that the only way of getting this right is to get paint matched to the tail piece decal. Is that what you have done ?
I've not been too active recently but hoping to put engine in frame this weekend. So maybe photos then. It doesn't help that the frame is in a different location to where I live. Still haven't sorted the cam pin problem though.

[/quote]

Hello HWG27, on this set all the lines and the blue is painted.
However; if you would buy the stickerset from Piki he will give a spall piece extra of blue sticker to match your paint to. That is easier and the blue on Piki's stickers is very close to the original blue.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on April 03, 2018, 08:51:54 PM
Huw, here's a better photo of Menno's excellent work. You can see the centre tank section has been perfectly matched to the tail decal. The lacquer looks like glass  :).
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on April 04, 2018, 08:31:33 AM
There was a time when 'Paint by Dream Machine' was a USP for a bike sale ...I guess this has been superseded (sorry Bita RIP  :-[ ) by 'Paint by Menno Dek'. A much deserved moniker for Menno's work IMHO. I reckon we owe it to JamesH too for alerting us to Menno in the 1st place  ;D ;D
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on April 13, 2018, 12:57:54 AM
Huw, here's a better photo of Menno's excellent work. You can see the centre tank section has been perfectly matched to the tail decal. The lacquer looks like glass  :).

Looks fantastic.
Little progress from me as a flat/dead car battery stopped me getting my engine reunited with it's frame until today.
I called in at Allbike Engineering on the way to discuss the rocker pin problem and the general consensus was that the rockers/tappets were good, so as you'd suggested, leave 'em in.

[attach=1]
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on April 13, 2018, 07:48:58 AM
Huw, good to see progress is being made and the rockers don't need to come out, the pins really are hassle to get removed, not a DIY job unless you have a stick of dynamite and understanding neighbours!
It appears as though you have used Athena gaskets, same as mine although I have used a genuine head gasket to be on the safe side.
I'm taking the bike for its MOT tomorrow so fingers crossed.
Oddjob, You will not be disappointed with Menno's work, it's awesome. Don't use Parcel Monkey to send the paintwork, they charged me another £35 after initially charging £28 saying the parcel weighed 38kg, it weighed 12kg, I weighed it before collection. Don't use DHL either, they delivered the parcel to someone 1/4 mile away and left no card, this is in London and I had to play hunt the parcel. Both Menno and I were pulling our hair out trying to find where it had gone.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on April 13, 2018, 08:27:25 AM
I always use My Hermes International when sending tanks etc to Menno. Touch wood, never had a problem so far.
Title: Re: CB250RS Rebuild Progress
Post by: Rozabikes Tim on April 13, 2018, 02:52:43 PM
I always use My Hermes International when sending tanks etc to Menno. Touch wood, never had a problem so far.
Hermes international! Most deliveries we get from them are out the back of owner driver Vauxhall Corsas... what's the gig with International - Austin Ambassador? ;) ;D ;)

Seriously, top tip though Julie, expensive kit to go missing when done.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on April 13, 2018, 03:20:41 PM
I always use My Hermes International when sending tanks etc to Menno. Touch wood, never had a problem so far.
Hermes international! Most deliveries we get from them are out the back of owner driver Vauxhall Corsas... what's the gig with International - Austin Ambassador? ;) ;D ;)

Seriously, top tip though Julie, expensive kit to go missing when done.
Our My Hermes lady has an Astra Van, very up market. Menno always arranges the return journey for the precious goods. I just arrange the goods going from here to Menno.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on April 14, 2018, 03:50:23 PM
Hmm, no MOT today for the bike. I begun to fill the tank and to my dismay the fuel tap has developed a really bad leak as a result of not be used for 10 years.
Unfortunately nobody makes a rebuild kit for this particular tap and I've been forced to order a new one from CSMNL at an eye watering €119!!!! S/H ones on ebay are going for £30 and are untested. I hope they deliver quickly, spring appears to have arrived  8).
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on May 05, 2018, 06:50:40 PM
It's finished  ;D.
I fitted a new gearbox driveside seal last week and put the engine back together earlier this week. Everything now back on and adjusted.  I took it out today for a 50mile shakedown and all is good. Only weird problem is the neutral light staying on in second gear, not sure why, it was OK a couple of weeks ago? Otherwise all good. Thanks for the words of encouragement when the seal blew and it was an engine out job >:(.
Thanks to Piki for fantastic decals and to Menno for being a wizard with the spray gun. I'll keep the revs to 5k for the first 300 miles and then slowly build them up to 6.5k until the 500mile service.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on May 05, 2018, 07:15:48 PM
That is lovely, such a stunning colour. There are going to be lots of Honda SOHC owners taking it easy for the next few weeks, what with all this running in of newly built engines !!!.
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on May 05, 2018, 08:19:38 PM
I always felt they were a good looking bike with nice proportion,  set off well with the twin pipes. Fabulous job you've done in bringing this one back to life.

It looks great. Hope it all runs smoothly now and you've got the bugs out of it.
Title: Re: CB250RS Rebuild Progress
Post by: Spitfire on May 06, 2018, 11:04:28 AM
Great job, it looks really nice, I always liked the looks of those bikes.

Cheers

Dennis
Title: Re: CB250RS Rebuild Progress
Post by: haynes66 on May 07, 2018, 08:14:58 AM
well done mate, youve done a cracking job there. enjoy the ride.
Title: Re: CB250RS Rebuild Progress
Post by: Piki on May 07, 2018, 09:24:54 AM
Congratulations mate, very good job!!!  Fantastic bike!!
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on July 18, 2018, 03:20:37 PM
Generally looking around today and found the following

https://www.ebay.co.uk/p/Honda-Cb250rs-Cb250-RS-Xl250s-1979-1981-Engine-Crank-Rod/27007292260

Could be of interest.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on July 21, 2018, 10:47:52 AM
Thanks Nigel. I've just returned from a few days in Devon to find the new main bearings have arrived from Simply Bearings. Just waiting for the con rod kit to arrive and will then take the crank to Bedford for a rebuild. I'll keep an eye on the link you sent in case the worst happens!
Dave
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on July 26, 2018, 09:11:50 AM
In the interests of trying to avoid a repeat, it's always horrible to see someone's work in building their engine result in damage like this,  thought I'd offer a constructive view of what appears to have gone wrong.

The question I posed about the plug,  I couldn't see from the photo what condition the electrode was is.  What I was looking for may need a magnifying glass to inspect the round plug electrode for signs of melting as this can induce pre-ignition conditions that would lead to failure.

As I understand it,  a holed piston results from early or pre-ignition. Meaning,  that the whole mixture has been spontaneously ignited before the maximum ignition advance point, in effect trying to make the motor go backwards while it's being spun forward by mechanical connection to road speed. This is normally always catastrophic. Heat builds veeeeery quickly and passes above melt point of the components in seconds if consistently applied.

The plug electrode is just one source of this,  if too hot it will glow and and can ignite the mixture as compression comes on during the phase leading up to the engine's set timing.

As Bryanj pointed out,  there may be a question mark over ignition accuracy which has the same potential if too much advance is applied. This should be easy to check for by making sure any timing marks are accurate and strobing it to verify.

Basically,  you either need compromised ignition or a very hot component that will glow hot enough to ignite mixture to get to this status. Another source can be the exhaust valves.
From the photos it looks as though the exhaust cam lobe has scoring / wear visible,  this would indicate that the engine has lost valve clearance on the exhaust valves. If they're kept even a very small amount off their seats they will be unable to lose heat and can get hot enough to give you pre-ignition.
Inlets are cooled by incoming cooler charge so don't generally do this.

If you look at the head photo there appears to be a site of combustion between the exhaust valves and the edge of the head. It would probably take less than 30 secs running to hole the piston in these conditions.
Running conditions were also against you as I believe it runs a accelerator pumped carb? So leaner constant running and richer during throttle twisting.
Now it should have no problem running at that mixture,  but if you've already got a mechanical compromise it'll give you no safety leeway regarding cooling the combustion chamber from throttle application.
It just looks like it lost exhaust valve clearance ( assuming the plug electrode is not melted)
It'll probably be ok if after rebuild if you can identify what happened and take steps to avoid the same conditions.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on July 27, 2018, 05:51:26 PM
You have made a lot of good points Nigel.
I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning.
The mark on the head are fine particles of the piston.
It could be the ignition module as you and Bryan say, the only   answer I can provide on that is its the original ignition module before I rebuilt the bike.
Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?
My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?
The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!
Thanks for you help  :)
Title: Re: CB250RS Rebuild Progress
Post by: HWG27 on July 27, 2018, 06:41:59 PM
You have made a lot of good points Nigel.
I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning.
The mark on the head are fine particles of the piston.
It could be the ignition module as you and Bryan say, the only   answer I can provide on that is its the original ignition module before I rebuilt the bike.
Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?
My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?
The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!
Thanks for you help  :)

I've only just caught up with this and seen the photos in the other thread. What a shame that is. I've only ever done that with 2 strokes and that was both times a simple high compression/low octane incompatibility. It's  a very unusual thing to happen with a bike like that.  I have a spare CDI lying around somewhere that I know to be good, and you'd be quite welcome to try it. These thing do though tend to either work or not. I would not have thought it possible that the CDI would suddenly advance the ignition for instance. I'd be looking at the mixture tbh. Pattern piston should be fine imo. There's plenty of wiseco xl250 stuff about that you could use, although you pay a premium.
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on July 27, 2018, 07:02:21 PM
The 400 wasn't the cdi unit that went wrong but the flywheel generator and they just went bad for no apparent reason
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on July 27, 2018, 08:42:46 PM
On the 400N and the the CX 500 the ignition advance is done by pulser coils whose output modifies the 'normal' trigger waveform. I was told it was based on a Maserati patent. On those bikes the CDi Unit, pickup coils and high voltage output on the alternator (which charges the capacitors) go bad. I don't have the shop manual for the CB250RS to compare but it wouldn't surprise me if the RS doesn't employ a similar design. What people do on those bikes if the alternator HV coil goes bad is to fit a 12v to around 200v inverter which then powers the CDi. The only disadvantage being that you need a charged battery to start the bike.

[attachimg=2]

Chapter from CB400 Honda manual here:-

[attachimg=1]

Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on July 31, 2018, 10:13:56 AM
That's interesting Ash, I wondered how they worked to advance the timing.

I was struggling to get my head around that schematic on the right as it looked like the timing was being retarded to me. Then realised I had a vision of clockwise rotating crank as my reference,  on which you'd have to move to the left to advance  :)

The timeline,  indicated with the arrow,  is going to the right and starting from TDC is effectively extending to earlier in the crank cycle.

Would it be true then that losing that high voltage pulse would stop it advancing?  As that's what it looks like to me.
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on July 31, 2018, 12:52:07 PM
You have made a lot of good points Nigel.
I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning.
The mark on the head are fine particles of the piston.
It could be the ignition module as you and Bryan say, the only   answer I can provide on that is its the original ignition module before I rebuilt the bike.
Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?
My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?
The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!
Thanks for you help  :)

The oil question you make is understandable but I feel that as soon as you get a hole in the piston it'll blow the oil out through any route it can.  In other words, the oil loss in a consequence of the piston failing and was still present in the sump until this point.

It's worth it I think splitting the details down to more closely examine what the sequence of events was leading to failure.

But first to look at piston tolerances. I believe right from the early years of Honda engines they have used specific metallurgy in the piston to create a very low expansion rate. It enabled them to run from the outset with a very tight bore clearance and not suffer from undue wear or seizures. This also makes for a very quiet engine for this aspect.  In addition, keeping that clearance really tight giving a very uniform and precise oil film separating the two. This is most advantageous in heat transfer from piston to bore and avoids the piston accumulating too high a heat load. It's this set of conditions you are trying to create during rebuild and hence the comments about original piston spec in determining what to machine the bore clearance to.

It doesn't so much impact the melted piston crown as there are other factors at work. It may be surprising that all aluminium alloy pistons are not able to resist the combustion temperature within the engine.  They are primarily protected by boundary layer insulation that prevents the heat from reaching the metal surfaces. That is conditional on the combustion process and it's rate of burn. The combustion is not an instant "explosion" but starts from the plug and moves through the flammable mixture,  in reality it's a fairly soft event that allows the boundary layer to remain intact which in turn protects the piston materials.
This is where it gets interesting. If you lose control of that process and it does explode, then it shocks that boundary layer into non existence and so removes the protection that ordinarily exists. Any melting of the piston crown (which is of course not subject to friction) confirms that has happened inside the cylinder. We can divide it further. Detonation is a post ignition event that usually gets to the periphery of the piston and starts to melt the area above the ring lands,  it doesn't usually get the centre. But it can heat the plug up until it glows,  hence my question about plug tip melting. If this happens, it opens up the route (glowing plug) to pre-ignition.......which will definitely melt the crown in its centre.
So, if you have no evidence of detonation? And the plug shows no sign of being high enough temp to glow,  then you are looking at another very high heat source to set the mixture off in pre-ignition.
The only other part that would give a consistent risk like this are the exhaust valves.

The feeling you'd have got when riding the bike is almost nothing if the above is true. You'd be going along feeling everything was normal,  but the piston crown would be moving toward melting.  Probably start flexing like a trampoline until it blows itself through as it fully melts. You wouldn't even know until it failed and would probably happen within about 10 secs. It's an unusual failure but with quite a precise fingerprint. Hence why I feel the exhaust valves have initiated it by getting too hot,  and suspecting you've lost valve clearance on the exhaust side.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on July 31, 2018, 12:59:47 PM
I just live3 the way you explain thing Nigel. Even as a 'novice' with a little knowledge, I understand everything you have written there and as usual, I have learnt something new today (unfortunately at Dave's expense !!!).
Title: Re: CB250RS Rebuild Progress
Post by: AshimotoK0 on July 31, 2018, 02:40:34 PM
That's interesting Ash, I wondered how they worked to advance the timing.

I was struggling to get my head around that schematic on the right as it looked like the timing was being retarded to me. Then realised I had a vision of clockwise rotating crank as my reference,  on which you'd have to move to the left to advance  :)

The timeline,  indicated with the arrow,  is going to the right and starting from TDC is effectively extending to earlier in the crank cycle.

Would it be true then that losing that high voltage pulse would stop it advancing?  As that's what it looks like to me.

Yes I think loss of those pulses would result in loss of the  advancing function Nigel and I don't think there is any feature within the electronics design  to detect that the advancer pulses are not present. Certainly a neat design though, let down by poor encapsulation ... someone at Hitachi should have invested in a vacuum chamber to de-gass the potting material. I was told last week that CBX1000 CDI's start oozing black liquid after a period of time..that's a new one on me.
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on July 31, 2018, 04:16:30 PM
Thankyou Julie. I think it's horrible to see someone's hard work fail like this,  but even worse would be a repeat so I think it's worthwhile to discuss.

Ash,  it doesn't look in that case to have contributed to the end result as I see it as it would have to advance to get something like this to happen.  Worth verifying on reassembly I guess but failure to advance should result in restricted revs and relatively safe operating conditions, just a frustration that it wouldn't accelerate as it should.
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on July 31, 2018, 04:20:10 PM
Ash 650 units leak goo as well
Title: Re: CB250RS Rebuild Progress
Post by: Rozabikes Tim on July 31, 2018, 09:47:01 PM
I just live3 the way you explain thing Nigel. Even as a 'novice' with a little knowledge, I understand everything you have written there and as usual, I have learnt something new today (unfortunately at Dave's expense !!!).

The motorcycling equivalent of Brian Cox me thinks? Excellent read.
Title: Re: CB250RS Rebuild Progress
Post by: paul G on August 01, 2018, 08:19:41 AM
Yep excellent read and very informative  :)
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on August 01, 2018, 07:06:58 PM
It's a subject that obviously goes on its way in all of our engines but usually we don't have cause to delve that deep unless it fails.

Thankyou for the kind words. 

There's so much knowledge held with the people on here generally which makes it so interesting for those working on these bikes nowadays,  hopefully my thoughts can complement that.

Nigel.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on August 01, 2018, 07:57:01 PM
Thank you for such a great insight Nigel, a great read.
Based on your knowledge of what probably what happened deep with my engine I have steered clear of the pattern piston route. I have ordered today a +0.50 oversize genuine piston and rings from CSMNL. They are over three times the price of pattern parts but in the end more than worth it if only for piece of mind. The only genuine part I couldn't get was a gudgeon pin but I have a brand new pattern one of those I can use.
Using genuine parts also allows the tighter clearance to be used (thanks Graham for your advice also).
I have bought a spare barrel off ebay for £10 just in case and a CDI box as a spare although I'll use the old one as there really was nothing wrong with it prior to the restoration, the bike ran perfectly prior to laying it up for 6 years. I have been going through everything with a fine tooth comb following the big bang. I have stripped and checked the gear clusters after cleaning in the parts washer. It is only when checking with a strong magnifying glass when I found tiny bits of alloy from the piston sitting in the roots of the gears. Thankfully there is no damage to the gears whatsoever.
The upsetting conclusion to this is I built this bike for the o/h to use following the passing of her test late last year. This bike was to act as a stepping stone to her graduating to the 400/4 I restored last year (and still with zero miles!) but unfortunately she cannot get on with the kick-start. With summer rapidly passing and not knowing when I'll get the engine finished due to working away from home so much she has bought a Yamaha YBR250  :o from ebay (or rather I did) to keep her riding confidence up.
I dont know what will happen to the RSA when it's finished, I've spent a small fortune on it and I'll not get anywhere near my money back if I sell it. Luckily I retire in four weeks time at a young 56 so at least I'll have plenty of time to polish it (and retrim a VW bus and a Sunbeam Alpine for friends, oh, and start bee-keeping and learning to play bagpipes in London!).
This is one of the greatest forums, long may it continue.
Dave
Title: Re: CB250RS Rebuild Progress
Post by: MCTID on August 01, 2018, 08:48:09 PM
Great thread following a devastating end to your hard work on the renovation. I have to admire your fortitude and 'never say die' attitude. Some brilliant analysis from other Forum Members who have obviously given the matter a great deal of thought.....one of the many reasons why this Forum is just so bloody good. Hope all eventually works out OK for you. Lastly, as a 'victim' of a %^&*($ Piper waking us up every morning at 'daft o'clock' in Woolwich Barracks many years ago......I would sincerely suggest that you do NOT take up the bagpipes - unless of course you wish to lose every friend you ever had, and every friend you may ever hope to make in the future !

Hope your rebuild goes well.
Title: Re: CB250RS Rebuild Progress
Post by: Trigger on August 01, 2018, 08:59:37 PM
It's a subject that obviously goes on its way in all of our engines but usually we don't have cause to delve that deep unless it fails.

Thankyou for the kind words. 

There's so much knowledge held with the people on here generally which makes it so interesting for those working on these bikes nowadays,  hopefully my thoughts can complement that.

Nigel.


I haven't had time to sit down and read this thread and take it all in until now. With other things going on and my head rattling with other work, i have just realised it is a RS and not a N. ( Reminder to myself, PAY MORE ATTENTION)
Interesting reading Nigel. I would be looking very closely at the state of the exhaust valves as a culprit. Are the valves too thing on the edge ? This is a common problem with this engine if the valve face has been over re faced or over lapped.
Honda did have a few issues with this unit and i can remember that one was the heat distribution from the chamber.
I would love to see the valves and valve seats in detail . 

I would not be re building this engine until i found what the fault was in the first place. Until you find the cause, it will do it again. 
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on August 04, 2018, 10:45:49 AM
More good advice Graham. The exhaust valves appear as though they have covered a few thousand miles following the rebuild and not 460! They were ground along with the seats using a Serdi during the original rebuild. They needed cleaning up again with a light grind this time to remove the bits of melted piston on the faces.
Based on both yours and Nigel's advice I have a new genuine piston and rings and have now ordered a set of new exhaust valves although these are pattern valves as originals are not available. I ordered a new Athena gasket kit yesterday. Newman Cams sent the repaired cam back on Wednesday (they only received it on Tuesday, great service and only £25).
Hopefully I'll get the crank and barrel back the week after next to start the rebuild.
Alan, thanks for the advice re:bagpipes, noted, I'll go with the second option, the violin. Or maybe the third, a trumpet 🤔😊
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on October 28, 2018, 04:20:11 PM
Well, the weather has turned and we appear to have gone from summer to winter in two days!
Now it's cold and I have at last received the rebuilt crank back from the engineer last week I have commenced the engine rebuild. All is going well with the exception of one of the crankcase 6mm bolts deciding to snap whilst I was torque setting it at 8lb! Luckily I managed to remove the broken section without having to split the cases again and without causing any thread damage. Why the bolt sheared I do not know, maybe the re-zincing process made it brittle or it had reached its yield point? Anyway, I replaced it with a new spare bolt and it was fine.
The crankcases are now together with Hondabond used to provide the seal and the balance mechanism all timed.
Hopefully tomorrow will see some more progress.
The Jota, Morini and Triton photos are for Alan (MCTID) who asked for some additional Jota pics but I don't know how to attach photos to a PM!
Title: Re: CB250RS Rebuild Progress
Post by: SteveD CB500K0 on October 28, 2018, 05:44:29 PM
You can attach photos (or any other allowed document) to a PM. At least you can using Tapatalk. It’s a while since I’ve logged in on a PC!


Sent from my iPhone using Tapatalk
Title: Re: CB250RS Rebuild Progress
Post by: MCTID on October 28, 2018, 06:27:26 PM
Great to see your 'collection' of other magnificent machines. Thanks for posting. With regards to Bagpipes, I saw an old film today about the Duke of Windsor playing the bagpipes and a Woman commented 'Its a good instrument to take up because nobody knows whether he is playing them well or not'

Tickled me anyway.....LOL.

Good luck with the rebuild.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on October 28, 2018, 06:48:47 PM
Thanks Steve, I'll have to go onto taptalk.
Alan, great comment  ;D.
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on October 28, 2018, 07:08:02 PM
I like the Triton leaning against the wall nonchalantly in an "Ogri " style.

Title: Re: CB250RS Rebuild Progress
Post by: Allington (Steve) on October 28, 2018, 10:48:57 PM
Done the Tritton thing again so many times in my head over the years not making my mind up just which way to go but I have space here and I’m pretty sure it will happen at some time very soon.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 08, 2018, 05:22:40 PM
Engine now rebuilt and back in the frame. I completed 60 very cold miles today starting the running in process all over again! All appears to be fine so fingers crossed the genuine Honda piston won't melt at 480 miles as the pattern one did  :(!
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on November 08, 2018, 06:06:59 PM
Fingers crossed it will all be ok, great job.
Title: Re: CB250RS Rebuild Progress
Post by: Seabeowner on November 08, 2018, 06:07:59 PM
Great, 10/10 for persistence and a lot of hard work.
Title: Re: CB250RS Rebuild Progress
Post by: Bryanj on November 08, 2018, 07:01:32 PM
Well done again, i really hope its all ok now. I liked those when they were new, fantastic handling and a decent turn of speed for a small bike---but then my first bike was a 250 Royal Oilfield all 7hp of it!
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 08, 2018, 07:13:25 PM
Thanks all and thanks for all the advice during the strip down. The weather for tomorrow is dry so maybe I'll get some more (4000rpm =40mph 😴) miles in! I'll take it up to 4,500 cruising when I clock up 100 miles. I'll keep the revs to 5000rpm until I reach 500miles and then increase by 1000rpm for every 100 miles thereafter. I'm just conscious of another potential meltdown if I don't go easy during the running in period.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 20, 2018, 09:34:58 AM
Here we go again....... I need some expert advice from someone! 🙄. I very, very carefully rebuilt the engine with all new gaskets, new exhaust valves, checked the valve guides (all good) and fitted a genuine Honda +50 piston and ring set  with a rebore. After 260 miles the engine has used 700ml of oil, this cannot be right 🤔. The engine doesn't smoke on tickover but I asked the o/h to follow me on her bike last weekend and she told me it was smoking blue smoke all the time after pulling away. Sure enough after swapping bikes I could see it smoking. It does it all the time so it doesn't appear to be valve guides.
Am I just being paranoid, should I give it another few hundred miles to settle down or is this an indication it has been rebored incorrectly? I'm 100% positive I don't have a broken ring. The engineer who did the rebore took it out to 0.8 as he said the clearance given in the Haynes manual is too tight for a large single cylinder air cooled engine?
I ran the bike in for the first 100 miles at 4500rpm and have now increased it to 6000rpm when accelerating through the gears but cruise at 5500rpm. The Honda manual states 'Do not exceed 80% of the rev range for the first 1000 miles'. Having the original rebored (DSS pattern) piston sieze up after 480 miles despite carefully running the engine in I have been even more careful this time but still making sure I am not labouring the engine.
Is it normal for one of these engines to use this amount of oil and smoke like a chimney during running in, I know all the old CB250RSA Road tests advise keeping a careful eye on the oil levels but considering this engine only holds 1.5litres of oil the difference between max and min is only 400ml, that's about 100 miles at its present oil consumption! 🙄
To say I'm cheesed off with this bike at the moment is a true understatement, it's rapidly getting to the stage where it will be thrown into the back of the garage to rot😡.
Title: Re: CB250RS Rebuild Progress
Post by: Trigger on November 20, 2018, 09:46:17 AM
Not really understanding the piston to bore clearance that you have had done. Are you saying that you have fitted a 0.50 mm and the engineer has bored it to 0.80mm ?
Title: Re: CB250RS Rebuild Progress
Post by: hairygit on November 20, 2018, 09:52:37 AM
I hate to say this, but what Honda specify for clearance is right. It may well seem tight to anyone used to working on lesser bikes (Brit stuff, Harley's etc) but Honda alloys used in their pistons are specifically formulated for low expansion rates. The clearance it has been machined to may actually be on the service wear limit, or worse, exceed it. I'm willing to be proved wrong, but I'm sure Trig would agree with me on this. 700ml of oil is an awful lot to burn so quickly. How much pressure is in the cases because of blow by? How does the oil look/smell.

Sent from my X6pro using Tapatalk

Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on November 20, 2018, 11:50:33 AM
I didn't understand the tolerancing numbers either,  can you give more exact figures that it's been bored to?

It's certainly not a good feeling with something wrong again with so much invested in it.  Hopefully it can be be determined what is wrong.

I don't think it'll be a risk to run it if you keep a very close eye on the oil level,  it's unlikely to seize with oil going through at that rate, unless it runs dry.

It does look like it's going past the rings if constantly smoking,  but as hairy pointed out, what is happening with the breathers? Are they verified as clear?

If compression is ok? It would suggest the combustion is not pressuring the cases. If the oil rings are letting oil pass upwards,  then it could be tolerancing that has stopped them sealing.

Can you get a compression reading from it when warm?
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 20, 2018, 03:48:54 PM
The clearance I'm referring to is the piston to bore clearance. I sent a copy of the Haynes Manual clearance details to the engineer. This gives a clearance between 0.01—0.04mm but, the engineer said that was incorrect and the clearance for a large Japanese air cooled engine should be 0.08mm, for British engines he bores much larger to provide the required clearance. As Hairy says, Honda clearances are tight and I remember both K2—K6, Trigger and Tomb mentioning this in an earlier post. I think I have therefore answered the original question, it's been over bored. I don't have a compression tester but I know it's pretty good, I can feel it when kick starting it (no electric foot on the RSA).
I don't want to run it anymore and risk a seizure. Luckily I have a spare barrel I bought for £10 on ebay following the original seizure and this is on standard size. I'll remove the head and barrel on the engine and check it. If it is bored oversize I'll reuse the new piston and get the barrel bored to suit the 0.5 oversize. Although the rings have only covered 200 miles I'm not sure if I should renew these or reuse with the rebored spare barrel?
Thanks for the advice so far.
Cheers
Dave
Title: Re: CB250RS Rebuild Progress
Post by: Trigger on November 20, 2018, 04:29:28 PM
I would do the clearance to 0.02mm on a oem Honda piston and 0.03mm on a aftermarket one. 0.08mm clearance is brit bike clearance, shot the engineer  ;)
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 20, 2018, 05:02:17 PM
Thanks both. I'll take the barrel off, measure it and give the engineer a call if it is overbored. I'll keep you advised.
Title: Re: CB250RS Rebuild Progress
Post by: hairygit on November 20, 2018, 05:06:13 PM
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again. And if that IS the case, name and shame him please, so nobody else has the same issues!
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on November 20, 2018, 07:02:31 PM
It's difficult to comment on the engineering experience that leads someone to make an arbitrary decision about something like this.  I wouldn't be inclined to bet against the shear range of Honda's R+D efforts though.

The basis of these pistons, and metallurgy that's involved, is one of the principles of all Honda's road bike engines, and the design concept.

Conventionally pistons had been sized to allow clearance for the piston to expand into and fit the bore with a running tolerance once up to temp. In other words,  the piston expanded more than the bore so you had to predict the end result and machine cold to accommodate that process.

I don't know if they pioneered it? But Honda's development with the piston suppliers used high silicon content aluminium cast pistons to control (along with design structure) the expansion, and make it more or less the same rate as the cylinder structure.  Meaning that the cold and hot tolerance is virtually the same. 
It's this aspect that people inexperienced in them fail to account for.......or don't believe is true.

https://en.m.wikipedia.org/wiki/Hypereutectic_piston

The above link gives a broad overview of why this happens in discussing various piston material compositions.

One set of tolerancing criteria just cannot be overlayed onto another metallurgical scenario,  or certainly not without a throughly investigated process.

It has effectively produced a "worn out" bore as far as tye numbers tell us.


Title: Re: CB250RS Rebuild Progress
Post by: Trigger on November 21, 2018, 08:18:19 AM
I see over bore a lot and a lot of my work is correcting others mistakes  :o I still scratch my head when i see a barrel that has been honed wrong without a good cross hatch  >:(

As i have said before, it doesn't matter if a engineer is George Stephenson, a aerospace engineer, race or rally, if he doesn't know, he doesn't know  ;)
Title: Re: CB250RS Rebuild Progress
Post by: Tomb on November 21, 2018, 09:07:13 AM
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again.

So true!

I remember the conversation
http://www.sohc.co.uk/index.php/topic,16637.msg140753.html#msg140753 and the comments made appear to have been found proven unfortunately.

I have worked on lots of race engines that have been modified, mostly crank case work for bigger liners, race transmissions/clutches, and am amazed at how badly some of it is done by so called experts. All our engines are machined by us now.
As Trig says - cross hatching is essential for bedding in.

Lastly I would check you have the oil rings seated in the piston correctly, if they are 3 piece oil rings they can be fitted wrong easily.

Title: Re: CB250RS Rebuild Progress
Post by: Trigger on November 21, 2018, 03:02:40 PM
Whatever you do, if it is overbored, do not let him get his hands on the other barrel, anyone that ignores recommended tolerances and makes such a balls up cannot be trusted ever again.

So true!

I remember the conversation
http://www.sohc.co.uk/index.php/topic,16637.msg140753.html#msg140753 and the comments made appear to have been found proven unfortunately.

I have worked on lots of race engines that have been modified, mostly crank case work for bigger liners, race transmissions/clutches, and am amazed at how badly some of it is done by so called experts. All our engines are machined by us now.
As Trig says - cross hatching is essential for bedding in.

Lastly I would check you have the oil rings seated in the piston correctly, if they are 3 piece oil rings they can be fitted wrong easily.


I don't think it would mater if the oil ring is wrong at 0.08mm. I did advise 0.8 thou to 1 thou and, at 0.08mm that is over 3 thou :o That is service limits on a Honda SOHC  ;)
Title: Re: CB250RS Rebuild Progress
Post by: Tomb on November 21, 2018, 04:03:35 PM
The reason I say check the oil rings is even at the bore size he has now, its still at the top end of wear limits, and an engine running at these clearances wouldn't be expected to use that much oil.

And its always one to check rather assuming its not the fault.
Title: Re: CB250RS Rebuild Progress
Post by: Trigger on November 21, 2018, 06:05:33 PM
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o
Title: Re: CB250RS Rebuild Progress
Post by: Allington (Steve) on November 21, 2018, 06:41:36 PM
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on November 21, 2018, 06:46:03 PM
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
🙄🙄🙄🙄🙄 There's always one.....Steve 😀😀😀
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on November 21, 2018, 06:51:39 PM
The oil rings are the three piece type. I fitted these in accordance with the Haynes Manual and followed the diagram as in: first thin ring, second the scraper ring making sure the ends did not overlap and thirdly the top ring. The ends of the rings were set at 20mm apart to avoid them being in line. The oil rings were followed by the middle and top rings. These had the 50 stamped on them on one side only and these faced the crown of the piston as per the manual with the gaps spaced equally around the piston to avoid blow by. The three part oil ring had no markings but I did check with a watchmakers eye piece just in case but found nothing. These are all genuine Honda rings.
I assume all the rings are therefore fitted correctly?
I was hoping to venture into the garage today but was dragged out to start Christmas shopping 🎄🙄, do John Lewis do rebores 🤔?
Title: Re: CB250RS Rebuild Progress
Post by: hairygit on November 21, 2018, 06:58:57 PM
Probably best to see if Trigger will do the bore for you!

Sent from my X6pro using Tapatalk

Title: Re: CB250RS Rebuild Progress
Post by: Allington (Steve) on November 21, 2018, 07:00:18 PM
Very true Tomb, never assume anything when it comes to engineering  ;) I did have a engine in sometime ago where the owner had fitted the rings as they were stacked in the packet  :o

Better than that other bloke who left half the bits at home  ;)
🙄🙄🙄🙄🙄 There's always one.....Steve 😀😀😀

 :'(
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 11, 2019, 06:59:20 PM
Well, after a lot of faff the bike now runs and is much, much quieter than before!
In the end I bought a s/h barrel from ebay for £10 rather than throw a £100 genuine Honda ±50 piston and rings into the bin. I had the barrel vapour blasted as the paint was in a bit of a state and repainted it using PJI satin black (although once cured in the oven it is more matt than satin). I had the barrel bored to the middle tolerance as specified in the Haynes (and Honda) manual. The engineer cried 'no, it will sieze' but after explaining the Honda metallurgy methodology he relented and did what I asked. He warned me when he has bored previous CB250RSA engines to these clearances they have siezed as they run very hot. He also said the ones he worked on previously were dispatch bikes with pattern pistons so I guess they might well have siezed if subjected to london's cut and thrust traffic from the word go!
Im not counting my chickens yet given my previous two experiences but I ran it for 42 miles, it was amazingly quiet (the previous oversize rebore must have introduced piston slap of another level!). The best thing, it used no oil whatsoever, previously even after 42 miles the oil level would have dropped slightly!
The only slight downside is I noticed a slight weep coming from the end of one rocker shaft, oil weeps from these are a common problem on these engines. It wasn't there before so I'm at a loss why its strarted now? I know I'm not taking the rocker cover off again to remove the rocker shafts and replace the o rings. What should be an easy job isn't on these engines due to the stupid pressed in dowls being impossible to remove without a spark eroder!
If anyone has any suggestions for an alternative potential cure I'm all ears 👂😊.
Thanks for all your previous advice, hopefully it will run as it should from now on 🤞!
Tomorrow I'm also collecting the CB250RSA holy grail.... A genuine 'Hondastyle' nose fairing, as rare as hens teeth to find in undamaged condition.
If anyone has a set of front engine bars they wish to sell please let me know.
Title: Re: CB250RS Rebuild Progress
Post by: JamesH on February 11, 2019, 08:11:35 PM
Good work. Just checking - you using mineral or semi-synthetic oil?
Title: Re: CB250RS Rebuild Progress
Post by: Rozabikes Tim on February 11, 2019, 10:26:10 PM
Sounds promising. Bet you are relieved..... :)
Title: Re: CB250RS Rebuild Progress
Post by: K2-K6 on February 11, 2019, 10:44:15 PM
Cautious optimism then, hope it's going to run without issue this time.

To be ultra cautious regarding running hot if that's the case. Worth considering the plug temp grade given that it holed a piston in one of it's previous builds.
Overheating the plug tip can lead to pre-ignition which puts the piston crown at risk.  I wouldn't run the lower range 7 NGK choice but either 8 or even 9 if that's in the manual for this engine.  Don't know what you've got in it but the higher numbers will give you more safety margin if they run ok.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 12, 2019, 12:49:14 PM
I have an 8 fitted at present Nigel but may go up to a 9. What I am making sure of doing is planning my route where I don't sit in traffic. I'll stick to riding around the Chalfont/Amersham and Aylesbury areas until the engine is properly run in.
James, I'm using mineral oil, I don't think I'll run this engine on a semi synthetic. As the engine only holds 1.5litres of oil, runs hot and works hard I'll change the oil every 750 miles to give it (and its owner!) an easier life.
Title: Re: CB250RS Rebuild Progress
Post by: Nurse Julie on February 12, 2019, 02:03:04 PM
You deserve for it to be OK, you have persevered above and beyond. I hope it keeps going well for you.
Title: Re: CB250RS Rebuild Progress
Post by: Laverda Dave on February 12, 2019, 02:50:21 PM
Thanks Julie. My fingers and toes are crossed this time!
I cannot belive how much quieter the engine is though following the latest rebore, its a different bike!
I'm sitting on a train on my way to Westenhanger now to collect the fairing. I think I'll be sending it to Menno for a respray to match the tank etc he painted last year. Just wish I could find the engine bar.
SimplePortal 2.3.5 © 2008-2012, SimplePortal