I was installing my MI rail and noticed that the only way to align my barrel nut was to either under torque or over torque. When I get to 30ft-lbs, one of the notches is covering the opening for the gas tube. The wrench clicks for 80ft-lbs before I’m able to move it far enough out of the way to make it usable.
Anything I can do about this? It’s a BCM lightweight 16" barrel on a BCM upper receiver.
There are a couple of different ways do “fix” this issue. Well three actually.
Do the “torque and release” method whereby you torque (never exceeding your target torque value) and then release it as many times as is necessary till you get alignment.
Barrel Nut shims… good idea… hard to find parts… I make my own.
Sanding/filing/machining a few thousandths from the barrel extension hole. (Do NOT do this unless you can do the math! It’s VERY easy to remove too much material!)
Have a NICE day,
TJ
Moly grease on the threads and the above mentioned torque/release/repeat. Also try a different barrel nut.
I had this exact problem with my NSR barrel nut. I had a long way to go in order to index the barrel nut properly while maintaining torque range and was suprised the torque and release method worked. I thought I was going to need a new barrel nut. I used a good amount of moly grease on the threads and a enough on the barrel bushing so the barrel slides easily into the upper receiver.
As above, in my order of preference:
torque and release and torque again (you were already using grease to start with, right?)
if it’s a GI nut or you otherwise have spares, try a different nut. Generally not an option with proprietary nuts for free float forearms, though you can always call up the maker and they may send you another nut for free or cheap to see if it torques more easily.
I’m not a machinist and have never seen shims offered for sale. Great idea, wish someone would offer them, haven’t seen it. If you have the ability to make them, try it.
Sand the front of the receiver very lightly. This should fix it (as a last resort) and I have used this once with installing a proprietary nut that had the same problem you describe - not indexing in the 30 to 80 ft-lb range. It works. Go slow, check often. If you’re trying to do a precision build you would want the receiver front perfectly perpendicular to the bore, which probably won’t happen if sanding by hand.
Say screw it to the indexing thing, choose one of the newer monolithic style designs (VLTOR VIS, AeroPrecision COP and MEGA KMK) with a nut that doesn’t have teeth and doesn’t require indexing, it just torques to a stated value, typically in the 45-55 ft-lbs range, and you’re done.
The only time I have ever seen a shim on a barrel was a factory built Colt 6540
I find this extremely difficult to believe. Perhaps your wrench is out of calibration? It’s not like we’re talking a lot of rotation to get into a tooth groove. I’d suggest finding a qualified armorer in your area to help you finish installation. There’s no reason to use shims on a barrel installation. If you have to, something’s out of spec and I’d rather send it back to BCM for replacement.
Try another nut, first. Otherwise, Brownells makes a receiver face mandrel that you use with lapping compound and chuck in a drill. Usually just barely enough to break the ano will do it. Don’t go too far.
First I would say make sure your torque is ft pounds and not inch pounds haha it happens.
Check the mating surface of the receiver and the barrel extension make sure they are clean, I like a light film of oil but only personal preference.
Torque, break torque and re torque to 40 ft pounds if not aligned move up to no more than 80 ft pounds
If still no dice. Check with the rail manufacturer for any possible defects of the barrel nut.
Since BCM has a good track record for making outstanding products I wouldn’t be lapping or modifying anything.
If any of my remarks are not comfortable for you do your homework on a gunsmith that does good work with ARs and have them do the work
I keep a few barrel nuts around just because of this issue. QA has slipped over the years and sometimes I buy what I think is first quality from Brownells or Midway and it may take several barrel nuts to get one to line up correctly. Same with the threading on upper receivers. Sometimes they are just off enough to scare you using the wrench.
I’ve assembled a few hundred uppers at my last employer and it does happen. Annoying as hell when it does. Not as bad when it’s just a plain GI barrel nut where I could just go to the spare parts and try a few until one works. With some of the rail systems that used their own nuts it was a bitch.
It doesn’t mean anything is out of spec, just that both the upper and the nut could both be at the large end of the spec. The plus and minus that every spec has (that’s what a tolerance is by the way, the plus or minus of the dimension of a part, not how tight or loose the parts fit together… that’s a side effect of extremely loose tolerances)
How much do you need to exceed the max torque to get it to work?
This same topic is running strong on another site, dedicated to 3g competition, brianenos.com, in the technical rifle section. While a lot of feedback is similar to what is posted here, namely one should be able to obtain gas tube clearance while remaining in the recommended torque range of 35-80 ft-lbs, a respected poster stated that he has had no loss of accuracy with higher torque, within reason. This was also confirmed by a respected poster here, Robb Jensen in a PM.
After conversing with these helpful folks, I left my rebuild alone, having obtained alignment at 85 ft-lbs. A take away from the discussion at be.com is that removing anodizing via lapping is a bad idea as it opens the possibility of galvanic corrosion between the barrel extension and receiver face.
Mark
Well, I loosened and retorqued to 80ft-lbs 20 times. It got a little better, but it seemed like it stopped making progress. I added a small witness mark and saw that it stopped advancing after 9 or so “re-torques”.
I ended up torquing it to 84ft-lbs according to the electronic torque wrench from work, I’m sure it’s about 90ft-lbs+ when you take the mechanical advantage of the adapter into effect. I torqued it and took off the nut to inspect the threads on the receiver and the nut, doesn’t look like anything was damaged.
Mine torques right at 30, and lined up, right when it clicked. In fact, truth be told, its ever so slightly past where it needs to be, but the rail seems to be pretty level. I still know it bugs me, so I’ll end up fixing it
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You mean above 30 in torque, or not properly aligned? lol, if not alligned straight why’d ya not just fix it and get it centered? Seems silly to leave something so easy to do off center.
PS I think it was IG who mentioned he did the 3x method, but did not use grease until the 3rd and final torque. I could be wrong as I can’t find the thread but prett sure I recall him saying thats how he did it.
I did the 3x method, ima try a couple more than that again. In essence, with the wrench at 30, the min required, it is so minute, that unless you run your fingers down the side of the receiver, you wouldn’t notice. I’m gonna redo it haha
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I did the 3x method, ima try a couple more than that again. In essence, with the wrench at 30, the min required, it is so minute, that unless you run your fingers down the side of the receiver, you wouldn’t notice. I’m gonna redo it haha
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It’s a truing jig and i have one. Worked great.
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Guys, I’m gonna eat some crow on this statement. I’ve currently got a customer’s upper (also with an MI barrel nut) and it just will not make it. 30 ft/lb is just past aligned and 80 is dead center of the next tooth. I’ve torqued and released more than a dozen times. I just got off the phone with MI and have another nut inbound. Hopefully it works…if not, we’ll try a little lapping.