Piston/Physics: inch gas piston same as metric?

[W.E.G.]

Is there any inch/metric difference when it comes to gas pistons?

[AR-45ACP]

You can use metric pistons in inch guns.If that's what you are asking.

[W.E.G.]

cool...thanks!

[EMDII]

Laid side by side, there ARE dimensional differences. My L1A1 (UE) piston rod is about 0.7mm (.027") LONGER than that of my StG-58 (FN-built).

Both piston heads mic out at 1.1cm (0.433"). the shaft of the StG has a slightly different countour (shoulder) where the piston return spring snugs onto the shaft.

Most importantly, you want to make sure you have some "dwell" between the aft end of the piston rod and the face of the breech bolt. I don't know what the spec is, but the rod should NOT rest on the face of the bolt-carrier when the rifle is assembled. Too much dwell batters the piston rod as it decelerates, and insufficient dwell, or contact between the rod and bolt-carrier face requires higher gas settings as the piston tries to overcome the at-rest inertia of the recoiling mass. And it's return spring, And gather some energy before it starts dumping all the momentum it has into the recoiling parts.

I garnered this info from a Brit who was an advisor to the Saudi Royal Army, at the same time we Americans were advising the SANG. He said he and his Tommy friends had done some 'interchangeability' looks at the FAL the Saudis issued, and the L1A1 they carried.

FWIW, my FALO-C (Israeli HB carbine conversion) reworked by George Gouger has about .05mm clearance. And my Israeli piston is about 3.2mm longer than that of the L1A1.