Actual piston size

[bhiggins2]

I am rebuilding my 2000 Dino engine. I have 2 complete engines and a third block. I have measured the cylinder bores on all the blocks and also measured the used pistons that came from those engines. New pistons are listed as 86.0, 86.5 and 87.0 mm. None of my used pistons match these numbers. Most are larger than 86.5 and smaller than 87.0 and some have almost zero wear if any on the piston skirt so it is unlikely they have worn off >0.3 mm. Can someone confirm whether the advertised piston diameter [86.0, 86.5, 87.0] is actually the piston or the bore for which the piston is to be installed? I live in the USA and don't want to order the wrong size pistons from Europe. Also, I have not found a piston to bore clearance measurement in the workshop manuals. What have other people used?
Thanks,
Ben Higgins
1967 FIAT Dino Coupe 2000

[Stefan2000]

I think, usually they mention the bore size and not really the piston size itself.
It also depends on where you measure your piston, on the top they are smaller in diameter compared to the skirt. Piston size is usually measured on the skirt.
With a rebuild you also can't go by the piston to wall clearance stated in the manual. New pistons come with documentation on the clearance, sometimes depending on how you are going to use the engine. New pistons are mostly (in my opinion unfortunately) forged pistons, which depending on type of alloy used require a larger piston to wall clearance that the stock pistons.
A wild guess would be 0.05mm for stock cast pistons and 0.10mm for a 2618 alloy type forged piston.