piston / bore size

I got pistons with 50 printed in the middle of them on the top. No other markings. I wanted to replace the rings.

who knows what size piston this is. standard or oversize ?

my calipers are measuring piston at 82mm which seams to be stock but i get a bore size of 82.3 approx. Dont know how accurate my calapers are they are only hand useable ones.
 
i found out from manual after reading this that you have to measure about 17mm up from the bottom of the piston. This has less wear. The top is actually smaller due to heat expansion. mine measure at 82.3mm so i am .5mm oversize. the stock should come it at approx 81.9mm.
 
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