Hi all, I have an M21 and have acquired a barrel for it as the original is corroded beyond saving. However, the piston is in excellent condition so I want to use it in the new barrel. The piston appears to be oversize as it is larger than the bore. To get it bored out by a local company what is the tolerance between the piston and the bore?

email (option): reddragonvapourblasting@aol.com

If it is a solid skirt piston .004"-.006" (measured across the thrust faces) will do it...Ian

email (option): ian@wright52.plus.com

I would say buy another piston!
No use in boring out a cylinder to a larger diameter if not needed. No sense in mending something that is not broken.
If prices in your place is anything like where I live it is not a choice that can be motivated by economy either.

82 mm i standard bore size, but be ware it may be unevenly worn, needs a thorough check with proper tools and know how before deciding to use un-bored. Ian and others can tell you more on this subject.

Also piston may be worn in ring groves. if not; in a couple of years when you have ridden the country up and down for thousands of miles and the barrel need a re-bore you already have the piston!

/s