Are my pistons standard or oversize?

[DayvW]

My pistons are all stamped with '1.0 mml 5' (I think it says).  Does his mean they are 1mm oversized?  I need new rings (typically the top one on all has a gap that is on the edge of the service limit) - and I was about to order some when it occurred to me that maybe the pistons are over size as on measuring they appear larger than the manual suggests.  I guess that stamp must mean something?  Any idea anyone?

[Trigger]

Pistons always show over size. It will say 0.25 or 0.50 or 0.75 or 1.00. It will depend on the manufacturer where this is stamped. If not sure measure the piston.

[mickwinf]

yes 1mm oversize.

[DayvW]

Super .. thanks - glad I asked! Any idea where I can get new rings (cant see 1mm oversize on David Silvers)?

[Trigger]

For what bike Davy?

[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?)?

[Bryanj]

The gap is the same as the cicumference is not much bigger for a 1mm overbore, also the oversize should be on the rings, usually at the gap on the top side on the opposite side of the gap to the manufacturers mark

[DayvW]

Thanks. I cant see any marking on the rings at all whether a manufacturers mark or an overize.  Think I will be safer with new rings .. all I need do is find some.  Any idea of a good source anyone?

[Trigger]

Davy, you dont want to be buying new ring's untill you know that the bore's are not worn out. Classic Motorcycles sell +1mm pistons , but i am not sure if they do ring's without the piston's .

[DayvW]

Thanks Trigger6969 - the bores if anything seem a little under 52 (more like 51.9) - am measuring with a Vernier gauge so difficult to be precise.  The pistons are marked as 1mm oversize (as confirmed above) - but also seem about 51.7- so allowing for my inaccurate measuring it all sounds about right (the pistons are snug - but not too tight (and she has been running well). The oil rings in particular seem very worn (the gaps are visibly way over .9mm) - which perhaps explains why the pistons were pretty mucky (carbonised stuff that was quite damp).  I hadnt noticed much oil burning when running it has to be said though - but with gaps that large I guess it would be?  I'm thinking perhaps I should fine someone to measure the bores a tad more precisely to be sure?

[Lynx]

Last time I looked Partzilla did oversized rings on their own. I has some. Have to wait for them to come from America though. About 2 weeks.