Recently rebuilt my friends 2000 cr125 top end, (new cylinder,piston,rings,ect).After starting up he complained no power nd low compression.Stripped it back down nd discovered that the cylinder the shop sent me was a "A" nd the piston "B".Everything looks ok,nd the shop when they checked with honda said this combo is ok.Anybody have any similar problems?Also the ring end gap is .020",is this ok?

The difference between the A and B pistons are only .0005 very small difference. Yep the ring gap is a bit on the large size at .020 but thats a std Honda ring for ya

So for what U have said so far, the bike should run at least as good as stock. Or was there another problem to begin with, before the top end rebuild??

Was the power valve cleaned and re-installed properly? Had a friend loose all low end from an incorrectly re-installed power valve. He corrected it and regained all low end.

Just replaced the cylinder,piston,nd rings on my friends 2000 cr125.After starting it up it lacked power,nd had a knocking sound.We stripped it down again nd found the new cylinder was a A size nd the piston B.I checked with the shop that sold me the parts nd they said they checked with honda who said this combo was ok.Also the ring gap was .020".Is this ok????

The .020 ring gap along with the piston mix up are NOT your problem. PV stuck?? or not opening?? old fuel?? slipping clutch?? reeds??......keep searching.

Thks for the help guys,now i know the cylinder,piston/rings are ok i'll start looking at the powervalve nd bottom end.

As long as it is honda original parts you use: A is bigger than B. Some aftermarket piston use reverse marking(B bigger than A)