I have rebuilt the top end 3 times now, the first one I put new valves and stem seals, I had the cylinder bored out and put a 1 over Namura piston and rings in. after running for approximately a mile it started smoking profusely and quit. After running a compression test there was none, after tearing it all apart again I found the Rings stuck down in the grooves. I ordered a new set of rings, measured according to the specs in the manual and it did it a second time. The third time I ordered a cylinder, piston, and ring kit. which ran probably 20 miles and it's starting to burn oil again. after putting this one together I realized that the first two pistons seemed very tight compared to the factory fitting, so I imagine the bore job was probably too tight even though it met what the manual said was within tolerance. I guess I'll run a compression test again but I was hoping someone might have an answer for me. maybe It just needs some more miles on it to get the rings seated in? This machine only has 900 total miles on it.