Need help with picking a new Piston.

[ford88mustang]

I'm in the middle of replacing my top end on my blaster. My caliper is tell me that my piston size is 66.25mm. And from what I see rocky mountain atv smallest size piston they have is 66.5. The dude I bought it from told me it had been bored 20 over. So it my caliper wrong and I should be safe buying 66.5 ? If so what should I get.

 

[Brute71]

look on the top of the piston, should tell you its size

 

[motoman9911]

Do what Brute said and you could also try getting a hold of another set of calipers if you think your set needs calibrated.

 

[ford88mustang]

I didn't see any thing on it.

 

[badams]

Have to clean off all the carbon off the top of the piston before you can see it. I use brake cleaner and a soft wire brush.