SOHC.co.uk Forums > CB350/400

Are my pistons standard or oversize?

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Trigger:
CB400/4 pistons are easy to get hold of but, not rings. Well, in the UK. How are you measuring the rings?. It does not work by putting them in the bore and checking the gap. Because if the bore is worn you will be getting a false measurement. 

Trigger:
You should have a bore size of 51mm + 1mm oversize. Piston length of 49mm and a compression height of 25.1mm.
For over size rings try Classic Motorcycles.

DayvW:
Doh .. sorry - for a 400-4 ('76).  I am indeed measuring them in the bore against the sizes quoted in the workshop manual (nb not Clymer as it has them wrong I have discovered!).  So - should the end gaps not be as quoted for oversize rings (ie. from top to bottom service limit of 0.7, 0.7 0.9) - does anyone therefore know what they should be (perhaps they're not worn then)?

Trigger:
The first thing you must do is make sure the bore's are 52mm spot on. Try measuring the ring in different parts of the bore to see what you get. Does  the bores have any cross honing marks? I am asking these questions because you cant always trust some people on boring. Some people will take the barrels to be bored without the piston's and ask them to over size by 0.25. The engineer will just measure the bottom of the sleeve and add 0.25 to it. Before buying new ring's, you want to make sure it is the ring's are the problem.   

DayvW:
Thanks for the insight - I will indeed try that. I dont think there was a problem per se (the engine was running fine - albeit odd transmission noises the subject of another post) - its just that as I'm stripping the engine I thought I'd check stuff and renew anything that appears worn (eg. as most valve ends (where the tappet taps) look damaged - some look like they have been re-ground(?!) then I'll replace them - and I'm thinking valve guides too whilst at it (gulp .. in for a penny ...).  So - on the rings I'll check all over the bores and see how they measure up (they certainly look in good condition).  So - would the end gaps for oversize rings be the same as standard rings - or would they be bigger (I assume larger rings would need a larger gap to expand into as they get hot?)?

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