CB250Rs Rebuild

[MRA464]

Thanks all for the 3rd wire info and all your advice.
Do let me know if my posts get too boring.

So my first real issues, I had a stripped out the frame and I was left with a seized centre stand tube and more of a worry the engine with a seized lower rear bolt, all others were free and left loose.
Try as I might I could not remove the stand, so I concentrated on the engine mount bolt.

It was the typical steel corroding in aluminium with added salt and general road dirt.

I tried the usual methods, lubrication, heat, force, time, more lubrication etc not to no avail. Fortunately for me after several attempts I managed to get the unthreaded part of the bolt to move,
So I again tried to get the bolt undone but all I was doing was causing it to twist.
A new plan was undertaken, lucky for me the engine case is two lobs rather than being completely enclosed. This gave me access to the bolt so with a dremel and small cutoff wheel I managed to cut most of my way through the bolt with out touching the case. Now the bolt was weak, so I just sheared it off using a socket on the head, happy days one side off.
I now cut through the protruding threaded side as close to the frame as I dare, ready for it to be drilled out.
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Carefully I centred a drill and drilled enough of the bolt away to allow me to remove the engine with no damage. I then continued to drill the bolt and eventually with a drift it came loose and popped out, Result.
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The center stand was cut out of the frame and will be modified or replaced in a later post.
Now I could strip the engine to see if I had a good one.
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Oil looked like caramel sauce, fingers crossed, as it was running.

[MRA464]

So now the engine was out I could take a look at its internals.
I had read in various posts that the threads easily stripped, so I had allowed the bolts to soak with oil overnight before attempting the disassembly.
Luck was on my side and they all came out easily with no issues ,moment of truth off came the cam cover.
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All looked good to me, but I am happy for your comments please. The rocker pads were clean with no pitting or damage the close up photos just show the wear lines in the surface and makes them look worst than they do visually.
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The bearing surfaces looked good again with no damage. So next to be removed was the cam chain, never stripped one of these so out with the manual, after some hassle I eventually manged to pull up the cam chain tensioned and pinned it in place. I had measured the tensioner are at approx. 7/8mm so I may change the cam chain on rebuild along with the balancer chain as advised by you kind gentlemen.
Cam sprocket was removed and out with the cam shaft, again it looked ok, bearing surfaces were good and the lobs showed some wear, closer examination with the camera showed some small imperfections, are these acceptable?
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At this point it was time to check the next know issue with these bikes, cracked head.

[MRA464]

The only issue I had removing the head was trying to undo the top sunken 14mm headed bolts, after breaking a 1/4 drive socket, I managed to find an old 3/8 drive socket that I could fit inside the recess and after some force it came loose and all was good, no breakages or damage threads.
The head was eased off and I turned it over to see what I had, HAPPY DAYS all looked good no obvious cracks. Even looked like a good combustion colour
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Now had it had a rebore?
The piston looked ok with no apparent slop although it looked oily. On cleaning I could see no marking so I assume it is original.
The bore was also very good with no steps or scoring, so it looks like I have been lucky and picked up a usable engine. ( hoping your comments don't dash this hope.)

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I will carry out the engine work New piston /Rings, Cam chain, Balancer Chain and a general good clean as part of the rebuild.
It needs the output shaft seal changing as well, not sure if this can be done from the outside or if it needs the cases split.
I will also change all the other external oil seals as I go whilst I am in the mood.

So lets get started on the rebuilding of this little classic. Lots of parts to order and find.
 

[K2-K6]

As you note, it's not quite the same looking at a photo compared to having the parts in front of you.

But the cam and rocker faces show considerable wear, the right side cam profile in particular looks like its been running with no clearance as the wear is complete round the base circle.  Also the oil has not been able to protect the surfaces as it should either through age or fuel dilution.  There's characteristic and noticeable "pickup" from the two metal surfaces being unsupported by oil film which has given the wear marks experienced.

It's not completely trashed itself,  but more hanging on with grim determination.

[Laverda Dave]

I agree with Nigel, the cam looks like it's begining to break up on the lobe.
These cams are made from chilled iron, replacements are virtually non-existant and very expensive if and when you can find one. The rockers also look as though they have started to pick up due to dirty or lack of oil.
I would send both the cam and the rockers to Newman Cams in Kent, they will advise if the cam can be saved. They will weld new pads to the rockers and grind them to match. Your biggest problem though wil be getting the rocker arms out of the head. The rocker shaft is held in the head by a dowl, they are an absolute pig to get out, they will break if you do what they Haynes manual tells you to do, grind a notch in the dowl and use a screwdriver to attempt to lever them out! I found someone who spark eroded mine out called Peter Lovell Developments, he's in the midlands. He may do yours, he may not as he said mine were a real pig. He only did mine becuase another company gave me his number! He does have a website though. You can still get the dowls from Honda.
What are the cam bearings looking like in the head, if you can feel any groves with your thumb nail that's not good news either.
As for a replacement piston, see if you can get a genuine Honda piston and rings if you can, they are available sometimes. I had to use a brand called IMD, no problems at all (second time round!). Make sure you only get it rebored by someone who sticks to the Honda specifications, I learnt the very hard way what happens when so-called experts do what they think is right and not what Mr Honda designed!

[Skoti]

Yep, the valve gear looks a bit tired.
I notice the tappet adjusters show also signs of pitting, are the valve stem ends OK?

I've seen worse though, but I suppose it would run on a bit longer in that state if you didn't want to throw extra cash at it.

Most of my recent projects have suffered "mission creep" where I've spent much more that I intended to, and more than the bike was probably worth...

Good luck though and please keep the photos coming.



   

[Laverda Dave]

You will have to split the cases to renew the output seal. Only buy a genuine seal from Honda, the one I had was part of a kit of seals for the engine, it tore and the oil poured out on start up, oh how I laughed . I am always really careful when I put seals on, plenty of oil on the lip, make sure it doesnt turn in on itself etc and still it tore (a tiny nick but enough to allow the oil to spurt out under pressure). When I compared the genuine item next to the pattern part it was obviously inferior, the rubber was very soft.
Splitting the cases now you have got this far isn't a big deal, you can keep all the gearbox shafts in the case. You will need three hands though when you renew the balance chain as the crank and front balance shaft is in one half on the case but the rear balance shaft is in the other case, you have to feed the balance chain around all three. I did it on my own but do yourself a favour and have a couple of trail runs before going for the real thing to give you the feel of it. To make it more of a challange you need to mark the balance chain links in three places with a bit of paint to ensure the links match the timing marks on the balance shafts. It is difficult but rewarding when you get it right.
Don't forget to use Hondabond on the crankcase joint after your practice runs and when you go to join them for the final time, a very thin smear will do.

[MRA464]

Oh dear guys that was not the news I was expecting, thanks for your replies, this is going to be a big learning curve.
It sounds like this build could get expensive.
This bike as a project was really just to get me out of the house and the bike back and running, rather than rusting in someone's garden.

As the bike is not going to win any shows and is likely to be run very occasionally, do you think I need to get the cams and rockers done straight away? I guess that it will just get noisy over time? I will defiantly get the bottom end split and do the internal work plus piston and rings, but might hold off on the head work and do it at a later stage, thoughts please.

I have also just bought another one in blue which is in much better overall condition and is also a runner, so I will split that one at some point. I hope that if all is well with it, I will get this up to a higher standard and keep it for investment.

I will continue to collect parts and see where we get.
Again thanks for all your comments and advice.

[Bryanj]

If it ran before it should run again albeit noisey and not at full output, only problem is you might take cam and rocker beyond repair but that shouldnt happen if plenty of good oil present

[Laverda Dave]

Take the rocker cover off the one you have just bought and check the condition of the head, cam and rockers. If better than your original engine it could be a case of mix & match to get a good one, very common back in the day when these were throwaway bikes.
I think Newman Cams charged me £27 per rocker and £56 for the cam reprofile about four years ago, so allow for a £100+ spend if you go this route. Try Peter Lovell developments first though to see if he can get the two dowls out. Have a go yourself first with plenty of heat and long nose pliars, you might be lucky and they might come out, you have plenty of time on your hands and its a learning experiance so worth a try, dont give up yet, plenty of help and guidance here so plough through it

[K2-K6]

As Bryan indicates,  it's been running and we've seen worse.

To separate,  obviously a route through to fully fix as described by Dave with recent experience of exactly the same is the most desirable to bring it back to technical quality. In that,  you've a well researched route and recommendations for suppliers too.

To run as is,  if it were for me,  I'd want to redress the rocker and cam surfaces to remove (as much as possible the peaks without taking off base material) just to give them better chance of stopping pickup,  one material transmission to the other component, and run with some consideration in allowing for it's recognised condition.
It appears that both the oil condition was compromised,  and it's run with nil clearance on one of the tappet too.
Running with clearance set at maximum tolerance should assist it's future lifing.
The oil,  obviously new is going to be used.  But I'd use a specific type,  VW diesel specification with code 501.01 as it was formulated for their series of engines that had conventional material in the valve train along with very high loading. Essentially it coped with this by using a tightly defined oil package that's of use here. It's particularly good at avoiding this problem. 

If you're not certain about cleaning the tappet and cam surfaces,  I'd offer to do that if you could get them to me.

[MRA464]

Thanks Guys for all your inputs.
I will get a price for the reworks and go from there, as stated I am in no hurry.

I believe these bike will become in a few years the next collectable so best to invest now.
I will upload pics of the new bike once I get it, I will then get the cover off that one and see what its like.

I may have two that need repair !

[Laverda Dave]

The good news is they are already becoming 'collectable' but I dont think they will reach giddy heights in terms of investment.
I rode mine to the Critch tramway event a couple of years ago just after I completed it. One of the stewards offered me £2,750 for it there and then! He wanted it because he was finding it difficult to kickstart his Vincents! Even though mine is a kickstart model he still wanted it, he really did want to do a deal there and then! I did offer to swap it for a Vincent though . Even £2,750 would not have come close to the  money it cost me to restore it. The simple fact is if you are intending to restore it to make it investment grade you really will have to spend a serious amount of cash and you will not get the money back unless you put the bike into dry storage for 20 years! I restored my bike to ride it and to have fun on and it does that in bucket loads, it always produces a big grin as it feels like you are going fast even when you are going slowly!
My advice is to restore it, ride it, change the oil every 500 miles (it's only 1.3 litres), clean it properly and have the fun on it that it was intended for. If you make a few pennies after a few years you'll be laughing, you won't make a fortune but you would have had heaps of fun along the way

[Laverda Dave]

Going back to the cases and replacing the balancer chain. The two photos will give you an idea how the cases can be split leaving the crankshaft and gears in the lower crankcase. The front balancer shaft is in the top crankcase half.
The second photo shows the new balancer chain fitted and its route between the two balancer shafts and crankshaft. The paint marks on the balance chain are there to ensure the timing marks are matched as it is easy to just be one link out and the balance will be all over the place!

[andy_c101]

Thanks Guys for all your inputs.
I will get a price for the reworks and go from there, as stated I am in no hurry.

I believe these bike will become in a few years the next collectable so best to invest now.
I will upload pics of the new bike once I get it, I will then get the cover off that one and see what its like.

I may have two that need repair !

I recommend checking the splines for any backlash, on the gearbox output shaft, for the drive sprocket.  Splines were completely shot on my resto project!