I finally got the piston off my ATC250R. I scraped most of the build up off and all I can see is "IN" would this be a stock piston? The sleeve is still very thick, so I think it is possible, but not sure. Anybody know? Thanks in advance.

 

The "IN" means "intake". It is marked to show that the piston faces the intake side of the cylinder during installation. Some stock and aftermarket pistons have this marking but not all.

There should be a number stamped on the side of the piston near the wristpin or underneath the piston that will identify it as a stock piece or not. Some of the pistons that Honda used were "ART" brand while others don't even have markings.

If it is a wiseco, the numbers will be etched into the top of the piston.

KNOWSALOT

 

It's an "ART" and on the inside wall there is a
1
2

does that mean anything?