robtmc Posted March 20, 2012 Report Share Posted March 20, 2012 Been out in the garage, stripping down my LR-308 of it's AP4 barrel, FF quad rail. Task is to install a new Criterion 18.5" barrel and Troy TRX FF tube. Bit of history: originally had a DPMS carbine FF tube on with 16" barrel, nut was a fine fit on the threads by hand. Easy to torque smoothly. Replaced it with a DPMS quad rail, and immediately notice the nut is much tighter on the receiver threads. Got it together, but was concerned the tight threads threw off the barrel torque setting, fighting the friction. Nut cam back off fine with the DPMS wrench, but not as tight as I thought it would be. Kind of supports my feeling the nut might not have been properly torqued.Now I go to put on the Troy barrel nut, and it is ridiculously tight, more so than the DPMS quad, so much that the Troy wrench (shaped like a "C") spread open enough to skip long before the nut was bottomed out. Squeezed the wrench back enough with a vise to remove the nut, but this is not right. I have seen pics of Troy wrenches that are closed, not open, but that is what Viking Tactical sent me.I have a PRI wrench that cannot spread being closed, but the center hole is too small to go over the barrel.Thoughts on this? About ready to put the AP4 barrel back on so at least I have a functioning rifle. Did wire brucsh the receiver threads, no burrs or anything, and the old DPMS nut still spins on easily.Addendum: Smeared some metal polish on the threads and working it back and forth. Still barely 1/8" of engagement before it get very stiff. You can feel the friction heating up the nut. This is a crock considering the cost of these parts. Troy of course is closed long before I found this, so no call to them yet. The fact that two DPMS nuts go on either freely or with a bit of friction tells me the receiver is withing spec, as I suspect that will be the first Troy response. Quote Link to comment Share on other sites More sharing options...
Slash Posted March 20, 2012 Report Share Posted March 20, 2012 It's likely that neither of the nuts is out of spec. The fact is that the specs for the receiver threads are different for each manufacturer. e.g. An ArmaLite receiver doesn't have the same threads as a DPMS or a RRA .308My advice would be to stop messing with it until you acquire the correct barrel nut. You're lucky if you haven't messed up the receiver threads. Quote Link to comment Share on other sites More sharing options...
Robocop1051 Posted March 20, 2012 Report Share Posted March 20, 2012 Slash beat me to the punch. It sounds like you have an Ar10 barrel nut and you're trying to fit it over a dpms receiver. If you're absolutely sure you have the correct nut, maybe you've been polishing the wrong part. Might be time to bust out the rotary tool and clean up those inner threads. Quote Link to comment Share on other sites More sharing options...
Slash Posted March 20, 2012 Report Share Posted March 20, 2012 A Dremel tool is no substitute for having the correct parts. I abhor those things. I've always said that Dremels should be registered and controlled, not guns. :) Quote Link to comment Share on other sites More sharing options...
robtmc Posted March 20, 2012 Author Report Share Posted March 20, 2012 The guy at Troy suspects the wrong nut too. The difference must be miniscule. He said the threading on a thread or two was a good indicator. Said the Armalite nuts used to have a small "A" marked on them. No marks on this one, so I hope this is the problem. New nut on the way.As far as I can tell with calipers, this thing should thread. Thread pitch, major and minor diameter all are extremely close. Suspect it is one of those % engagement things, where the drilled hole is slightly undersize to increase the thread engagement? Funny, as the two DPMS nuts I have are slightly different, one goes on by hand, the other needs a wrench to go fully on. Large diameter threads must be a problem to form.Would love to see the specs on the threads for both, and understand why in hell DPMS would choose some slightly different spec. No meaningful competitive edge to that. Quote Link to comment Share on other sites More sharing options...
robtmc Posted March 20, 2012 Author Report Share Posted March 20, 2012 Glad Troy is sending out a new nut. Best I can tell, the problem is the threads were too shallow. This is not the best macro shot, but the flattened tops of the threads are very clear under magnification. Explains the binding. Quote Link to comment Share on other sites More sharing options...
Tripledeuce Posted March 21, 2012 Report Share Posted March 21, 2012 Most commercial machine threads are machined for 70% engagemont. Its not un-commen to see flat tops on threads. Respectfully Terry Quote Link to comment Share on other sites More sharing options...
robtmc Posted March 21, 2012 Author Report Share Posted March 21, 2012 Yeah, the specs I found show a lot of room for the min and max on threads. Not at all clear, and in this case, seems there is no "mil-spec" for anyone to follow. Hell, maybe that is the difference between Armalite and DPMS, thread %? Thread diameter looks like 1 3/8", maybe -16.Crazy if so, why do that? Quote Link to comment Share on other sites More sharing options...
Slash Posted March 21, 2012 Report Share Posted March 21, 2012 The pitch is the same on both types, but the pitch diameter is different. You can't measure pitch diameter with a caliper or even a micrometer alone. You need thread wires and a chart.Flat tops on the threads have nothing to do with the issue. You'd be more likely to have problems with sharp peaks.Why DPMS choose to follow their own path is likely only known by the engineer who designed their barrel nuts/receivers. I'm sure that in his mind there was some good reason... Or, he might have been so 'smart' that he didn't realize what a headache he was creating for the rest of the gun world. lol Quote Link to comment Share on other sites More sharing options...
Tripledeuce Posted March 21, 2012 Report Share Posted March 21, 2012 or, he didn't care what a headach he was creating. Terry Quote Link to comment Share on other sites More sharing options...
Slash Posted March 21, 2012 Report Share Posted March 21, 2012 Most of those guys have so little real-world experience that they have no clue as to the consequences of their decisions. Quote Link to comment Share on other sites More sharing options...
robtmc Posted March 22, 2012 Author Report Share Posted March 22, 2012 Guy from Troy got back to me, no replacement nuts to send out: "Currently we do not have any more barrel nuts because we had an issue like yours"So they are aware of the problem, sounds like a batch of poorly machined parts. Stand up service, not arguing the point like so many companies. Quote Link to comment Share on other sites More sharing options...
Tripledeuce Posted March 23, 2012 Report Share Posted March 23, 2012 Personally, I'd chuck it in the lathe, and using a boring bar, just chase the threads a little. respectfully Terry Quote Link to comment Share on other sites More sharing options...
Slash Posted March 23, 2012 Report Share Posted March 23, 2012 Personally, I'd chuck it in the lathe, and using a boring bar, just chase the threads a little. respectfully TerryThat's great if you have a lathe and the knowledge to use one. I suspect that most who are reading this post do not. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.