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Old 07-15-2012, 09:55 PM
buckshot85 buckshot85 is offline
Join Date: Oct 2010
Location: Cowtown, Wis.
Posts: 2,732
Default gas cylinder/piston wear

how critical is the rotational wear in the piston "D" opening in the gas cylinder? does a little extra rotation hurt anything? a local guy has a couple that have a little play but the bores appear good. I have no gauge. is there a quick way to mic the bore?
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Old 07-16-2012, 11:23 AM
M14 M14 is offline
Join Date: Jul 2011
Location: south mississippi
Posts: 811

There is naturally a little play in the hole. It is not so much the rotational wear that would be the problem if it is not in spec. The problem would be the effects on the front end of the piston. The loose or oversized hole would cause the front of the piston to dig into the gas cylinder walls at the front and scratch it badly eventually. The hole should be .204"R minimum and .206"R maximum and the D flat should be .151" above the horizontal centerline. I got these measurements from the blueprints.

The most important thing is the inside diameter of the cylinder, and outside diameter of the piston.

Are you sure the piston's tail is in spec ? Use G.I. piston if you can. It should be .36 diameter.

I don't know any way to check the cylinder without a ball micrometer for inside diameters. Geeck brought his set of Starrett gauges up here for me to use for this. I understand that Kuhnhausen says that they used a two diameter plug gauge when making them at the factory to check them. I suppose you could have MSC (Manhatten Supply Company) or Pacific Tool and Gauge make you one. It would be .5000 for the go gauge, and .5006 for the no go. If you do go this route, always clean your gas plug threads perfectly and inspect for gouges or burrs, and use extreme care not to force or scratch the gauges.

Last edited by M14; 07-16-2012 at 11:32 AM. Reason: added care warning for gauges
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