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Bolt carrier/upper rail fit query

1267 Views 5 Replies 3 Participants Last post by  bamaak
The upper rails on the NDS M92 receiver I have fit the carrier very tight, measuring from .953 to .973 free space between the rails. The carrier measures .973, so I am not surprised to find it binding a bit. On my factory (Arsenal) guns I find the free space tends to run about .980. Before I take the file/dremel/grinder to the rails, what do you look for as a tolerence between the rails and the carrier? I see the thickness of the rails is different on each side, and it looks like my extra width is on the right rail (looking from stock to muzzle) at this time.
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GPSIG, I've built a couple from flats and bent blanks, and so had to trim the rails. I'd like to offer some observations.

But first, I'm not sure if you have the rifle built or are noticing this issue while test fitting loose parts. I'm going to assume it's built.

The clearance you are curious about varies between rifles some. And is more loose on factory rifles than I would have thought. I can do some measuring for you if you wish, but I'll throw out .020-.030 as a guess. The total range out there is probably much wider. My builds have been much tighter without a problem.

This clearance can vary along the length of the receiver. I think it's good for it to be less at the area near the trunnion. However the bolt will affect how the carrier fits in this area. Be sure to test with the bolt in place (with no recoil spring, also) to determine if it binds when going into battery. Sometimes the bolt will cause the carrier to shift hard against one side rail.

Which brings me to the uneven thickness of your rails. This not a common thing, I don't think anyway. Consider the possibility yours might be a factory defect (not likely). But if you have binding at the point of going into battery it could be more likely.

If you decide to work the rails, I'd use a good quality mill bastard file (hardened receiver, need a good file) and use long smooth strokes. Debur and polish too.

If you forced your front trunnion rivets into their holes, and didn't file slightly until the rivets easily insert into them, some crazy things can happen when the rivets are crushed. Twists or bows that can lead to issues. Seems like forcing the rivets in can "pre-load" the receiver to respond in some way. My theory is a little bow on the trunnion combined with a tight fitting carrier leads to a bolt stem that pushes the carrier into a bind when entering battery.

If you're test fitting loose parts. Wait til it's together to test again and fine tune if needed. (although your measurements indicate something must be done.)

I'm going to measure a new receiver sitting on the desk here.

BTW, is your right rail complete with the clearance cutouts along it's length like the left side? (the cutouts for installing the bolt and carrier) I know it's a long shot, but still looking for a QC issue. uneven rail thickness=a curiosity
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I just checked the rails on a couple of different mfg's 47 and 74 rcvr's. They are all very close on side to side width (both rails equal) about .015" to .060" clearance. batteries died but I can be more specific if you need. Just pm or post.
I realized measuring an unbuilt NDS wouldn't help.

A new unaltered 7.62 Saiga with 60 rounds fired:

receiver width at ejector-- .985

carrier side play--- .010-.012

An NDS AMD65 homebuilt, just sighted in, maybe 40 rounds.

receiver width at ejector--- .998

carrier side play--- about .020
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Thanks for the thoughts, pretty helpful in looking at the issue. Below pics show my rails are about the same size, and match a factory Arsenal to within .001. The difference from right to left appears to be on the carrier, which is a bit hard to see from the next two pics but the measurment difference between the two sides is .040 here, and about .030 on an Arsenal carrier. Using dychem it looks the rubbing is strictly on the right side rail, left side isn't showing any binding. My current plan is to take very small amounts off the right rail until it stops showing rubbing down the entire length of the rail. Based on how it moves now I don't think I have to do any more trimming to make the rifle work, but I would like to get the right rail to show a bit less friction. I can make the bolt hang up if I try (about half-way back while under hammer pressure), and can't do that on factory guns.

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With your dye chem work you know where it's rubbing. So you'll have it fixed soon.

Going slow when you start taking off metal I,m sure.

good luck.
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