Piston Cylinder size

Ok so i am going to rebuild the top end on my yz125 and what does it mean when the back of the cylinder is stamped with a B?

When the barrel was new, it was measured. There are four sizes of pistons. Yours took a 'B'.


If your barrel has never been replated and is still serviceable, a 'B' piston should fit it perfectly. But only if the barrel is untouched since new.

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