I noticed while decoking my M21 that there was one broken piston ring so I needed to replace it. Interestingly the piston is +0.080. I have never heard of this before and thought +0.060 was
as big as it got. The bike is registered 1958 but the engine is 1954 so this would be a cast iron barrel? Certainly the weight of it confirms this. Does anyone know if cast iron barrels are known for being bored out to this extent? The barrel bore is beautifully smooth and piston clean so I assume the re-bore was recent but unfortunately I got very little history with it. There were one or two other little problems, now more or less sorted, which would explain the price I paid - £4250. Lovely reliable bike though. Just come back from touring the west country with the missis. We had a grand time.
:) :) :) :) :)
Hi John.
My thoughts are if you can get a +.080" piston then its not anything that out of the ordinary.
Mike 8) 8) 8) 8)