Hello all,

I have the following issue when reassembling my engine:

I have an old, good condition piston and new piston rings to match. Old piston pin looks fine, so I am going to keep it. The cylinder has been machined and looks mirror-like inside.

When I want to install the top piston ring, it goes in fine, but it has some play in the piston groove... with a feeler guage, a .06 gauge goes in freely, but a .08 does not. The .06 gauge drags in some points (over the piston pin holes) but still goes in fine.

Does any of you know if there is any tolerances allowed there?

Should the piston ring fit tightly or snug in place?

Or is some "movement" allowed in the piston grooves? If so, how much?

Any input or comments is appreciated.

BTW, mine is a 400 cc, chain drive... so maybe tolerances might be different... or not?

Thanks,

RW

Piston rings fit tiny bit loose in the groove.

The measurement you need to check is the gap.  Put ring in cylinder (no piston) and measure the little gap between the ends.  Too much gap and you will get low compression and oil "blow-by".  Too tight and there is no room for thermal growth.

Also, cylinder wall should have a X pattern honed in and not be mirror smooth.  The X pattern helps to hold oil on the cylinder walls.