piston help

[honda380]

after much searching all i've done is confuse myself. someone please point me in the right direction. i need an '86 250r piston that is .50 over. the .50 is metric correct? what is the american measurement for it? and what is the overall diameter of the piston? thanks for the help. im confused...Corey

[Dave83]

You need a 67.25 piston.That is the mm (metric) measurement.Stock bore is 66mm,so if you are at .50 over(meaning .050 or 50 thousandths,on the sae scale.this puts you at 1.25 mm over,so 67.25.One mm is equal to .040.