Author Topic: CB250RS Rebuild Progress  (Read 12769 times)

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Re: CB250RS Rebuild Progress
« Reply #90 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.

Offline Nurse Julie

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Re: CB250RS Rebuild Progress
« Reply #91 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 !!!).
LINK TO MY EBAY PAGE. As many of you know already, I give 10% discount and do post at cost to forum members if you PM me direct.
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Offline AshimotoK0

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Re: CB250RS Rebuild Progress
« Reply #92 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.
“Alright friends, you have seen the heavy groups, now you will see morning maniac music. Believe me, yeah. It’s a new dawn.” Grace Slick, Woodstock '69 .. In the year of the Sandcast.

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Re: CB250RS Rebuild Progress
« Reply #93 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.

Offline Bryanj

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Re: CB250RS Rebuild Progress
« Reply #94 on: July 31, 2018, 04:20:10 PM »
Ash 650 units leak goo as well

Offline Rozabikes Tim

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Re: CB250RS Rebuild Progress
« Reply #95 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.
One day I'll have the time to restore it, not just talk and dream....

Offline paul G

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Re: CB250RS Rebuild Progress
« Reply #96 on: August 01, 2018, 08:19:41 AM »
Yep excellent read and very informative  :)
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Re: CB250RS Rebuild Progress
« Reply #97 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.

Offline Laverda Dave

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Re: CB250RS Rebuild Progress
« Reply #98 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
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline MCTID

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Re: CB250RS Rebuild Progress
« Reply #99 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.
Now: 2008 CB1300S, CB750K4, 1970 Bonneville. Various other 1960's 650 Triumph T120's/ TR6's/ TR6C's (all in bits...many, many bits unfortunately). Previous: 2007 CB600FA, 1976 CB500 Four. BMW F800ST. GS750E. ZZR1100. CB1300 (2). ZXR1200S. VFR800. CB750 Nighthawk. CX500. XS500 Yam. Suzuki GT500. BSA A10. Various Lambrettas. Zundapp Bella (honest).

Offline Trigger

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Re: CB250RS Rebuild Progress
« Reply #100 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. 
« Last Edit: August 01, 2018, 09:07:36 PM by Trigger »

Offline Laverda Dave

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Re: CB250RS Rebuild Progress
« Reply #101 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 🤔😊
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline Laverda Dave

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Re: CB250RS Rebuild Progress
« Reply #102 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!
1976 Honda 400/4
1977 Rickman Honda CR750
1999 Honda VFR 800FX
1955 750 Dresda Triton
1978 Moto Morini 350 Sport
1978 Honda CB400/4 'Rat' bike
1982 Laverda 120 Jota

Offline SteveD CB500K0

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Re: CB250RS Rebuild Progress
« Reply #103 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!


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Offline MCTID

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Re: CB250RS Rebuild Progress
« Reply #104 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.
Now: 2008 CB1300S, CB750K4, 1970 Bonneville. Various other 1960's 650 Triumph T120's/ TR6's/ TR6C's (all in bits...many, many bits unfortunately). Previous: 2007 CB600FA, 1976 CB500 Four. BMW F800ST. GS750E. ZZR1100. CB1300 (2). ZXR1200S. VFR800. CB750 Nighthawk. CX500. XS500 Yam. Suzuki GT500. BSA A10. Various Lambrettas. Zundapp Bella (honest).

 

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