well If 1/2 x 20 will work that is easy. I bet if it is cloes it will be fine. consitering that the piston should not be tight anyway and the pin retains it too. this means that I can make my own piston and tap it 1/2 x 20 and chase the bolt carrier and everything should work out fine.
I found the M12 x 1.25 to be less common. maby not as expensive as the 14x1xL , but not ijn my standard tap die kit. I like the idea of home made piston!!!! way too cool. would anyone be intrested instainless pistons if I make them ?? I guess I would have to get them to you for less than the ones out their allready, maby I can do that as long as the work remains fun. what if I could get a piston to you for $15 delivered. oh yes if I would sell one it WILL have the proper metric thread. ... Just thinking out loud. let ne know if their is ANY intrest, or how cheap it would have to be to not pass it up.. thanks