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Posted: 4/2/2015 10:25:51 AM EDT
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After sorting out the issue on my last build, I am doing another. This one is going to be an M-16A2 replica.
Here's my question. I went to torque the barrel nut down. From what I understand, MILSPEC is 30-80ft-lbs. I hand tightened it then set the torque wrench at 30ft-lbs. When it broke, the barrel nut was exactly half way between two of the recesses for the gas tube, blocking the hole in the upper receiver. So, I set the wrench to 40ft-lbs, it moved slightly, but was still not aligned. I did this all the way up to 80 ft-lbs, and it is still not properly aligned. I took it completely apart and repeated. same results. I took it completely apart and tried again...same results. I took it apart and se the torque wrench to 20 ft-lbs so I could determine where it breaks when I have proper alignment on the low end of MILSPEC. It breaks somewhere between 20-25 ft-lbs, which is barely better than hand tight. I am looking for suggestions on how to proceed. Short of cutting the offending "tooth" off the barrel nut, I do not know what to do. True advice and not a bunch of BS would be appreciated. |
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There are a few things you can try.
1. Repeat the tightening procedure using only one torque value (say 60 ft lbs) and tighten and loosen the barrel nut several times. When you do your final torque do not start at the minimum torque value, use a mid range value and watch your timing during your torquing. You may end up with it being timed correctly prior to reaching the torque setting. You can always check to see if you have achieved minimum torque by lowering the setting on the wrench and making sure it torques out prior to the nut moving. 2. Use a receiver lapping tool to remove a slight amount of material so you can properly time the nut 3. Use shims Gabe |
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First, leave it alone for a while. Go read something.
Never start cutting stuff............. OK! Back to the basic. Now take it apart and lay all the parts down. Clean the threads with a tooth brush. (male and female). Remove any grease, dirt, metal shavings, even small pieces of anything can interfere with getting the barrel tightened. Then hand tighten the barrel nut, feeling for anything in the threads. Undo the barrel nut and install grease on all the threads (a small amount goes a long way). Screw the barrel nut until it begins to get tight. Use the torque wrench to 30 to 35 pounds. Stop. Unscrew it and tighten it 1 more time to 35-40 pounds. On the 3rd time tighten it to 40+ pounds watching the hole for the gas tube hole. When it is lined up, install the gas tube. I have never gone over 50 pounds on any of mine. If it still doesn't tighten up then check for out of round threads on the receiver or the barrel nut for a bad thread. Each person has their on way they install a barrel, but the above is some general guide lines. Good luck. Ron |
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Option #4: Keep going.
While not the most advisable method, I can tell you that I've personally exceeded 80 ft lbs with no ill effects. I've also removed nuts from factory built guns that were clearly WAY over 80. In theory, the spec range should allow alignment. In practice, it doesn't always. If you have shims or are patient enough to wait for them, that is best. But if you have a decent amount of experience turning wrenches, a good set-up for securing the upper and don't feel like waiting on more parts to finish, a lot of us get away with over-tightening barrel nuts. Just understand that while the threads can take it, you could tweak the upper if it's not held properly and securely. Such as those pin blocks; I know some people use them to assemble uppers, but I never would. I personally find soft pine blocks optimal: 
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Quoted:
First, leave it alone for a while. Go read something. Never start cutting stuff............. OK! Back to the basic. Now take it apart and lay all the parts down. Clean the threads with a tooth brush. (male and female). Remove any grease, dirt, metal shavings, even small pieces of anything can interfere with getting the barrel tightened. Then hand tighten the barrel nut, feeling for anything in the threads. Undo the barrel nut and install grease on all the threads (a small amount goes a long way). Screw the barrel nut until it begins to get tight. Use the torque wrench to 30 to 35 pounds. Stop. Unscrew it and tighten it 1 more time to 35-40 pounds. On the 3rd time tighten it to 40+ pounds watching the hole for the gas tube hole. When it is lined up, install the gas tube. I have never gone over 50 pounds on any of mine. If it still doesn't tighten up then check for out of round threads on the receiver or the barrel nut for a bad thread. Each person has their on way they install a barrel, but the above is some general guide lines. Good luck. Ron Clean the threads, re-grease and try again. If that doesn't work, as stated, a little over 80 is OK. |
| Another option is to lap the upper with a lapping tool. You get the added (potential) benefit of better allignment of the barrel, especially if running a FFHG. Generally makes sighting in quite easy as well because the face of the upper is perfectly squared to the bore line of the barrel. I do this to all uppers I build, ounce of prevention and all that. In your case though it shightly removes material to get you that extra few degrees of nut rotation to be within spec on your torque. |
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Option #4: Keep going.
While not the most advisable method, I can tell you that I've personally exceeded 80 ft lbs with no ill effects. I've also removed nuts from factory built guns that were clearly WAY over 80. In theory, the spec range should allow alignment. In practice, it doesn't always. If you have shims or are patient enough to wait for them, that is best. But if you have a decent amount of experience turning wrenches, a good set-up for securing the upper and don't feel like waiting on more parts to finish, a lot of us get away with over-tightening barrel nuts. Just understand that while the threads can take it, you could tweak the upper if it's not held properly and securely. Such as those pin blocks; I know some people use them to assemble uppers, but I never would. I personally find soft pine blocks optimal: Sistysixdeuce: thanks. I don't mind waiting for shims, but I was unable to locate any..if I need to go that route. Does any one know where I can get shims? Also, I have stepped back and let it sit for a couple of weeks. No huge hurry, but I would like to get it done. |
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Did you use the recommended 3x torque method as outlined in the M16/M4 manual ?
If you don't know what that is..... 1) Grease receiver threads with approved grease (AeroShell 33MS) 2) Run up to 30 ft lbs and back off....do this 2x 3) Go for barrel nut alignment If this doesn't work, try a different barrel nut. |
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The barrel already has the A2 front sight post attached, so the barrel nut is the one I have on it. I got the barrel with the FSP and barrel nut already in place. Really don't want to take the FSP off if I can help it.
I will try the grease and 3x times you suggested. Thanks |
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Using a grease will make the torque higher than your torque wrench says, and not considered with the original 30-80 ft/lb spec.
I wouldn't go over 80, at all, with grease. You don't need grease. Many builders specifically don't use grease. Maybe that'll help? In fact, grease is its own debate. |
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Using a grease will make the torque higher than your torque wrench says, and not considered with the original 30-80 ft/lb spec. Wrong. Torquing the nut dry to any given number puts exactly the same amount of torque on the nut and upper as a lubricated fastener. What changes is the amount of pressure on the barrel extension flange; at the same torque value, the binding and galling of dry surfaces will result in less pressure against the barrel extension flange than with lubricated threads. Rest assured, you will not crush the hardened steel extension flange with any amount of torque you can apply. Lots of other things would let go first. And the maximum 80 ft lb torque spec exists so that the upper doesn't get tweaked; the threads can take several times that. If you have your upper clamped right, you can go way over 80 without a problem. But if you're using pin blocks or clam shells, distortion is probable. |
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