Piston Rings

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May 12, 2005
Hi Guys.

How can you tell what size your piston rings are +0.010 +0.020 etc.
the ones in my 1970 commando had no markings on them, so I assumed they were standard and bought some +0.010's because the gap was to big. When I came to fit the new ones they were way to small. I thought they would be marked.

Ron :(
The size of the piston rings will depend whether the barrels have been rebored and oversize pistons fitted?
Rings can be marked to show which way up they go but these marks can wear off.

Are there any markings on the top of the pistons?

I'm not quite sure if I have understood what you have said correctly but are you trying to compensate for wear by fitting oversize rings?

As LAB suggested the key to the ring sizing should be determined by the size marking on the pistons you are fitting the rings to! So if the bike is running 20 thou + pistons, you need to replace with +20 rings. However first I would try to determine how much wear is in the bore. The proper way is to internal mike the bore with a micrometer, the home workshop dodge is to get one of your old compression rings, seat this in the bore using the piston to ensure it sits square and compare end gap measurements at say 1/2" from the top of the bore, (this is where the bore wears most) and then down the bottom of the bore where the hone marks are still evident, I.E. where the piston rings have not been wearing. Normally 5 thou difference is the maximum allowable before a rebore & over size pistons should be used, rather than a re-ring job.
I tried this on my Trident, the method above showed 2 1/2 thou difference and after fitting new +10 thou rings, and honing the engine stopped burning oil visably and I took the bike from Sydney to Melbourne and back covering 3000 km without having to add oil. SO I suggest you try this before buying any more parts?

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