Bought a 'shooter grade' SKS a while back to do some work on, and found it had a problem I've heard some about; the action either doesn't cycle at all or only partially cycles due to gas leakage at the gas valve/gas tube junction. Just to try it, I put an o-ring on the lip where the valve body fits into the tube, and it cycled and threw brass like a frat boy tossing an empty bottle. For about 35 rounds, at which point the ring disintegrated. I checked around and found that new valves are hard to come by, so tried something. Took a piece of thin steel flat stock and cut off a thin strip. Bent into a ring that, when placed in the lip and the tube installed, would spring enough to butt the ends together. Had to try/sand/try to get a thickness that would allow the tube to fit without being too loose. It works great. The action doesn't cycle as hard as with a tight-fitting valve & tube, but I've got about 100 rounds through now with no failures, and aside from some powder fouling the seal ring is holding up fine. I'd think the thickness/width of the ring would need to vary according to the wear/corrosion of the parts on the individual rifle; in some cases a simple ring made of wire would probably take care of it, with some it would need a thicker/wider ring. So if you've got one of the 'rode hard and put away wet' rifles, might give it a try.