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I need help deciding what piston size to get; 66mm A, B, C....and so on. Piston to cylinder clearance tolerance is 0.079mm-0.089mm. My cylinder measures 65.989mm. ProX pistons come A-65.94mm, B-65.95mm, C-65.96mm, D-65.97mm, E-65.98mm, and F-65.99mm. To get a clearanance of 0.079mm-0.089mm I would need a piston sized 65.90mm by my calculations. Right? I know I'm missing something here but don't know what it is....
My cylinder is stamped with a B, but the existing piston is stamped C

Thanks

Here's a link to Wiseco's site with the ProX piston sizes. http://www.wiseco.com/ProductSearch.aspx

You might want to measure your cylinder again, looks like your calculations are right. According to the piston measurements you have, if you add the minimum clearance it would be to large for your cylinder, so I would measure again. An old carpenter told me measure twice cut once, that's good advice, in my case measure multiple times. good luck

I'm confident it's right. I didn't have a dial bore gauge so I took to machine shop that deals with engines, and they measured it. He measured it multiple times. Told me how much run out I have. Run out was only half a thousandth, and the cylinder is within the service limit spec of 66.1mm. So, the cylinder is good. Wiseco said the Wiseco pistons come one size, and they expand so you don't have worry about meeting the piston-cylinder clearance spec, because they're forged pistons. As long as your within the service limit of the cylinder of course. They don't have A,B,C...sizes like the ProX.

Honestly, bore size should not be less than 66.02 mm. More, possibly, but not less. Has it been re-plated?

You should have no problem replacing with the same piston that came out, or a forged replacement. Something does seem wonky with that bore measurement.