Help: Damaged Barrel Threads

[CJW]

I need some help figuring out if there is any way to fix a problem short of cutting and rethreading a barrel. Here goes...

My g/f wanted to change comps on one of her uppers. I threw it in the vice blocks, clamped it down, grabbed the armorer's wrench, and started to remove the old comp. It was tight but broke free okay and unthreaded most of the way without a problem. Then it grabbed tight again, tighter than it was to start unscrewing it at all. I poured some acetone on it in case there was junk binding it up and I was able to start turning it again after that, but it was fighting me every bit of the way.

The end result is it came off, but the last four or so threads on the muzzle end of the barrel are wrecked. The comp was warm to the touch from the friction of removing it and, I swear to you, it looks like the last four threads inside the comp melted into the corresponding grooves on the barrel threads.

My first thought was to grab an A2 birdcage I had laying around and see if I could thread it back on to unfoul the barrel threads, but that was a no go. The starter thread is so messed up I can't get anything to bite on it.

I know I can get it cut and rethreaded, but this is an 18" with rifle gas, so I really would prefer not to lose barrel length. My thoughts right now are to sand the damaged area smooth and go with the assumption that there is still more than enough thread to secure the new comp. Alternately, I was trying to think if there is any sort of jig or tool that I could clamp around the good remaining threads and unscrew that to try and clear/recut the damaged threads.

I am wide open to suggestions since my main option right now is to have a half inch or so of barrel lopped off. Any ideas?

[Stlhead]

Chase the threads with a die, more than likely there are plenty of threads left to keep your comp on.

[Shooter115]

I'd say you still have enough thread on there to hold the comp. Should be able to touch up the damaged threads enough with a triangular file to get a 1/2-28 die started. Don't try to re-use that comp without running a tap through it or you'll be right back in the same boat.

[dcloudy777]

I just had the same problem a few weeks ago, maybe not as severe, but the threads were definitely messed up. Chased it up with a 1/2X28 die and it was good to go.

[Agney5]

If you can't get it any gunsmith worth his salt should be able to chase the threads with a lathe. If that doesn't work he should be able to turn down the bad threads down to the minor. So you can keep your length and have fixed threads.

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[openclassterror]

Most gunsmiths under the age of 80 have a 1/2-28 die on hand, should be a $10 fix. If it is REALLY bad, have the bad threads turned down to minor diameter as previously stated. I have fixed more than a handful exactly as you described. FWIW, if threads start to get tight halfway off like that, stop IMMEDIATELY, douse it with oil, LET IT COOL and reverse direction. Acetone removes any trace of lube that was in there. What happened is called "galling" also known as friction welding. Not a bad plan to use anti-seize on comps, as they get pretty hot during use, and the threads get hammered every time the gas hits the brake.

[CJW]

Thank you, all.

First, for the life of me I could not recall that the complement of a 'tap' is a 'die', so at least I know now what the proper tool is called.

Second, I am a dumbass because I didn't even think of the galling issue—something that I am very familiar with when it comes to frame/slide contact on pistols, but which didn't even occur to me in this context.

I will call a couple of local 'smiths and see if they can't hook me up with a fix here.

[CraigS.]

CJW if the threads are really messed up like you say, it is possible for the die to not catch the correct thread engagement

when you start it. Be careful

[Brian Gonsalves]

One of these may work.

http://www.threadtoolsupply.com/murray-tools-thread-repair-system.html

[GunCat]

CJW if the threads are really messed up like you say, it is possible for the die to not catch the correct thread engagement

when you start it. Be careful

That's when your knowledgeable gunsmith can either pickup and chase the existing threads using the the lathe or turn down the damaged section (if the remaining “good thread” length is still sufficient), Several way to fix the problem and when you screw a new muzzle device into place no one will ever know what happened.

[Dan Sierpina]

One of these may work.

http://www.threadtoolsupply.com/murray-tools-thread-repair-system.html

No 1/2-28 thread.

[CJW]

My first plan is still to get somebody else to do this, but if I have to give it a try as a DIY job, what are the opinions on split-die thread chaser (like Brian linked) vs. thread file?

[Agney5]

If you van find one it is not a bad option, but the gun industry doesn't have a lot of standard threads. So finding one may be difficult. A thread file is probably a good bet for a diy project, hopefully you can get the threads at least cleaned up enough to run a die on it. Now go price all your tools if you don't have them already and then find a gunsmith and get a price quote. I'm not saying one will be cheaper but probably a good idea to check both options.

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[CJW]

Already priced. A 1/2-28 thread file (probably only good enough for a single use!) is ten bucks at Midway, and the dies seem to run about $30. So, if I can find somebody to chase the existing threads for less than $60 or so, it's worth it. If the quote starts much higher than that I will just take it as an opportunity to do some more hands-on gunsmithing of my own.

[Agney5]

Good deal, I'll be surprised if that is not close to what a smith quotes you. But it really is hard to tell sometimes.

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[LoganbillJ]

Thread files are very handy tools to have. I have fixed a bunch of threads that customers screwed up using them while working at a local cycle shop. They are good for more than one use as well. The set i have has multiple pitches (8 each)on each square rod. Good luck!

ETA: Enco has them for cheap.

http://www.use-enco.com/1/1/85845-made-usa-fix-thread-restorer-thread-restoring-file-sets.html

Edited May 7, 2014 by LoganbillJ

[openclassterror]

1/2-28 is not a gun thread. It is a Unified National Extra Fine thread (UNEF). Everything on the M-16/ AR-15 platform is a unified thread. Keep in mind that Eugene Stoner was an engineer for an aerospace company. Even the upper receiver to barrel nut thread is a standard Extra-Fine thread (1-1/4 x 18 UNEF), as is the buffer tube (1-3/16 x 16 UNEF). Most available dies for 1/2-28 are split dies, so you can loosen it, go over the boogered (technical term ) threads, snug it down on the good threads, and chase it backwards. That is the best approach if you want to save the threads. A thread file will usually have a 28tpi side, as a 1/4 fine thread is also 28 threads per inch (1/4-28 UNF). That would be my plan "B", if I didn't have a lathe. Brownell's has the split die for about $20. It is a good tool to have on hand if you play with ARs, and allows you to trade favors with your shooting buddies who eventually will need to borrow it.

http://www.brownells.com/gunsmith-tools-supplies/general-gunsmith-tools/taps-dies/dies/ar-15-m16-flash-suppressor-die-prod41592.aspx

Edited May 7, 2014 by openclassterror

[BitchinCamaro]

Next time, use a thin oil instead of acetone when things get sticky. Acetone may de-gunk as a solvent, but it leaves the metal surfaces nice and bare for galling.

[CJW]

Well, I have a split die and die stock on order. Any suggestions for oil when I am cutting? Most of what I have around the house is Militec or Friction Defense...

[openclassterror]

Grease is a better alternative to gun oils. Better yet would be diesel engine oil if you own a diesel. ATF will even work in a pinch, but most gun oils are too light (viscous) for cutting purposes.

[Shooter115]

My top pick is LPS Tapmatic in cream form. Gear oil works well, but it stinks. Since you are just cleaning up existing threads, I would say any old engine oil would suffice.

[BOOM]

CJW: You know openclassterror is the own ,operator, machinist of MOA Precision.. He knows

[openclassterror]

I use Tapmatic daily here, but you can't find it at any old hardware store. I was thinking stuff the typical home mechanic might have in his garage. Hands down the best tapping fluid EVER is Moly-Dee from Castrol, but it is WAAAAAYYYY more stinky than gear oil, and costs about $40 a bottle. I used Moly-Dee on a production job tapping 4-40 threads in stainless. Got 400+ parts per tap, vs less than 50 with any other tapping fluid available. Molybdenum Disulfide is the same friction reducer they use on some bullets to reduce barrel wear and increase velocity. Good stuff, until it's time to clean the bore.....

[uhcoog1]

I agree with the above- a simple die will work. Even if you lose some of those last 4 threads it doesn't matter. You've got plenty left.

[polymerfeelsweirdman]

you could try using a tap on the comp to save it

if that doesn't work have someone pop it in a mill and drill it out to major diameter

Edited May 10, 2014 by polymerfeelsweirdman