I have been pulling my motor apart and I was under the impression that it was 1641. On the inside of the piston I can't see 87 mentioned anywhere. The part number on the inside is 85ST80. Are these 85.5 mm pistons???

If so, I will be asking the shop who built the motor why this is so

the size is stamped onto of the piston scrap away the carbon deposits

That is first place I looked.....nothing there

I had another look.....you're right.

They are 87's after all