Further thanks to Righthand for posting that information as it leads me to adding something else.
Please note the minimum OD spec for a G.I. spec gas piston is .4970" and the maximum is .4975". Folks that entire spec is only 5 TEN THOUSANDTHS of an inch or a half of a thousandth of an inch, depending on how you want to say it. That is a tiny, Tiny, TINY tolerance for a gun part. That is also why it is expensive to manufacture a G.I. spec piston and why they cost what they do for a quality part.
I am always a good bit nervous when I get into the following because if one only uses a part of the info, it won't work correctly. It also relates fully ONLY to G.I. cylinders and pistons or at least G.I. spec. cylinders and pistons.
We separated G.I. gas pistons into a "size" for .4971", .4972", .4973", .4974" and .4975". IOW, there were five "sizes." We didn't use the minimum .4970" as so few G.I. gas pistons measured that size and we threw them in with the .4971" size. You also must remember we did not know or try to separate the G.I. gas cylinders by makers, we used all of them indiscriminately. We shot each rifle with different "sizes" of gas pistons until we found the one that worked best for accuracy in that gas cylinder in that rifle with that barrel in that gun. All testing was done in an extremely accurate and expensive test rack machine.
Usually, the rifles would shoot best with a piston somewhere between .4972" and .4974" BUT a very few shot best in either the .4971" or .4975".
We ALSO were using only three kinds of ammo: What is now Federal's Gold Medal Match, NM Lake City, and a handload we called "G4" through "G12" over the years as we slightly changed the load. We did not handload for every rifle.
There actually was a difference and sometimes a hugely dramatic difference in accuracy when we found the size piston the rifle liked best. I try to keep the different sizes of pistons in stock to use when testing NM rifles, but it is getting harder to do that as the parts dry up.
Now, please, Please, PLEASE consider this before forum members inundate Sadlak or anyone else for pistons that measure between that .4972" to .4974" sizes I mentioned. That "best spread" only works with G.I. gas cylinders and there is NO WAY to know ahead of time (before testing) what size your rifle will prefer. Further, different types of ammo and handloading procedures can throw off how valid that spread is.
If you have a Chinese or some commercially made gas cylinder, that spread may not be valid at all.
What I can say is that I've put G.I. spec pistons in all types of gas cylinders and they all worked well (if not better) for at least accuracy. So if you have one of the poorly made turned gas pistons or if it is not G.I., replacing it with a G.I. spec gas piston of either NOS G.I. manufacture or a properly new made G.I. spec commercial gas piston (like those made by Sadlak) will often if not usually help the accuracy of the rifle, but no one can say that with absolute certainty until it is tested in your rifle with your ammo.
What I do know is that I want an original or properly made G.I. spec gas piston in my rifle and any rifle I work on.