SOHC.co.uk Forums > CB350/400
CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE
Nurse Julie:
Mr Davo, Oddjob, Hairy, K2-6, Bryan and everyone else who have given their theories' and insights, I think your input is invaluable to this thread and please do carry on. This is a huge learning curve for me but maybe also of interest to other current members of this forum and future members' also. It would have been alot easier me doing this in 'private' ie, not sharing with anyone else but, what's the point in that......no one learns by my mistakes or enhances their knowledge base by something I get right. I still think the diagnostics are the most important part of this process as a means of hopefully preventing it in the future.
Thanks all for your interest.
matthewmosse:
I'm very much inclined to think the rings gaining a bit of rust in a prolonged lay up theory is why the rings would break up like that. On my 550 it had a indicated milsge that was not that high, - I cannot remember the figures exactly but around 30k, I doubt that bike had been around the clock given the wear elsewhere on it, but it had doubtless been stood a long while given the rust that had been painted over.
I suspect the rising values of Sohc bikes will see more bikes dragged out, forced to turn over then thrashed and flogged quick so if the theory is right we can look foreward to more of these threads. I've seen a few pistons where the rings have picked up on the cylinder, they do melt rather readily. I've also had a few where the piston have pre ignited and holed the piston, to me this looks like the damage from the bits chewing the head up. - the melting to me is as likely to be friction from repeatedly hammering those bits of ring til they exit and running temperatures combined. I did blacksmithing in collage and it was a trick of the old hands to take a bit of cold 10mm diameter steel, and hammer it so fast they could light a cigarette off it, so I reacon the melting might just be that. Rings are really brittle and easy to shatter so I can well imagine a bit of rust in the bores causing them to fail. Worst case of this kind of abuse I've seen was a dumper engine I bought a few years ago, probably easy start causing the top of the piston to fail ( they have a small pocket in the top of the piston with a spike inside it and a washer fixed around that to aid cold starting ) the washer had come loose off the piston, similar damage to the head to look at although the rings survived ok I think. It had obviously been run that way for a long, long time as the valves were bent and stuck plus the exhaust port was almost closed up with soot, crank though free turning was down to copper on the main and big end bearings. Con rod was a bit bent too. Just the impact seemed to leave the head and piston crown in that condition.
Craig CB400:
If it makes you feel any better Julie my 45k restoration project was equally mullered when I stripped it down and funnily enough mine also had 3 pistons and crowns damaged.
Mine had sat since 1997 and wasn't run for 9 years, promptly broke and sat for another 9 years before I bought it, just proves these things need to be ridden to be in good shape
Nurse Julie:
That's really interesting Craig, also good to see another wrecked engine ;D Can you remember if #4 piston was ok or was it one of the others?
matthewmosse:
Craig, you are right, these bikes are far better off being used, my cb500 got to 250,000 miles and the engines in better shape, just needs new rings and one selector fork plus gear it runs in.
Julie, you need to see the contents of my shed - plenty of wrecked engines in there, thanks to my buying at the very bottom of the market.
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