1984 ATC 250r piston size question

[hdd]

im currently rebuilding another engine for the 1984 ATC 250r and confused on the bore size the piston says 50 on the top of it but when I check it with my calipers the bore says 70.21mm and piston 70.15 on the Wiseco site its says 70.50 for 50 over. any suggestions why the difference.

[barnett468]

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your calipers may be off.

the clearance is correct, but the bore should be measured at 3 elevations of top, middle and bottom and in 3 different positions of of "rotation" or "clocking", like noon, 4:00 and 8:00 o clock or 0, 120 and 240 degrees.
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[onformula1]

im currently rebuilding another engine for the 1984 ATC 250r and confused on the bore size the piston says 50 on the top of it but when I check it with my calipers the bore says 70.21mm and piston 70.15 on the Wiseco site its says 70.50 for 50 over. any suggestions why the difference.

Keep in mind pistons are not perfectly round on purpose you may not be measuring it correctly.

[yaegerb]

Calipers get you in the ballpark but is no way to accurately measure a cylinder bore. You need a bore gauge for that. Take it to a shop and have them check it.

[hdd]

thank you for your help