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Thread: 1/2-28 AR muzzle attachment threads

  1. #1
    Boolit Master Tokarev's Avatar
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    1/2-28 AR muzzle attachment threads

    Wanting to do some experimenting with brakes, I bought a tap and die kit in 1/2-28 (thinking that once I am paying shipping anyway, getting a die just in case would make sense).
    The tap is perfect: it cuts very tight fitting thread to an AR muzzle. But the die cuts way undersized, in my opinion, thread: it's OD comes at 0.48" and a rod, threaded with this die, wobbles in the hole threaded with this tap.

    Is this normal at all?

    Previously I never used dies for the threads this fine and just never had a chance to see how snug they would be in a kit like this. Always had good luck with 4 flute taps though.

  2. #2
    Boolit Master



    M-Tecs's Avatar
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    Normally this operation is done on a lathe. Is it an adjustable die and if not what thread class fit is it?

  3. #3
    Boolit Master

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    Is it a "fixed" or solid die or does it have a slot thru to one of the chi Clearence holes? If it has a slot there shoud be a screw in the side from one side to bearing on other ide. This how yo adjust the dies to cut just what you want it to. You start it small and use the screw to open I ( spread it open ) until it cuts where it needs to be. This adjustment allow for many different thread fits to be cut with the same die or to compensate for when it starts to dull. The fixed or solid dies are just whatever you get and will vary from die to die and as they dull, there is no means to adjust them. They also vary as to material steels usually being a bigger thread than brass or aluminum. We had some set .015 big for threads that had to be ground after heat treat.

  4. #4
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    Dumb idea. MUST be done on a lathe or it won't be straight. Very exciting when a bullet hits the brake or worse yet a suppressor.

  5. #5
    Boolit Master Cap'n Morgan's Avatar
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    You would be surprised how much tolerance, depending on class, threads specs. will allow for - a tight fit is not necessary, and serve no purpose as the angular profile of the thread makes it pretty much self-centering, but the bearing surfaces on both barrel and brake must be absolutely perpendicular to the bore or you'll risk a baffle strike.
    Cap'n Morgan

  6. #6
    Boolit Master Tokarev's Avatar
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    Quote Originally Posted by Eldon View Post
    Dumb idea. MUST be done on a lathe or it won't be straight. Very exciting when a bullet hits the brake or worse yet a suppressor.
    Dumb or not, I now own a virtually free die just in case.

  7. #7
    Boolit Master





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    There is no way I would shoot a hand die, lathe held or not through one of my silencers. Good way to ruin something that takes a year now to process ,never mind the cost.

    Getting a barrel threaded correctly is not bad money spent.

  8. #8
    Boolit Master Artful's Avatar
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    Quote Originally Posted by Tokarev View Post
    The tap is perfect: it cuts very tight fitting thread to an AR muzzle. But the die cuts way undersized, in my opinion, thread: it's OD comes at 0.48" and a rod, threaded with this die, wobbles in the hole threaded with this tap.

    Is this normal at all?
    To actually answer your original question...
    On a class 1 sloppy it's normal
    on a class 2 or class 3 it's unacceptable

    1/2x28 UNEF class 3
    External Thread:
    Allowance 0.0000

    Major Diameter Max = 0.5000, Min = 0.4935

    Pitch Diameter Max = 0.4768, Min = 0.4740

    Minor Diameter = 0.4562 (see diagrams in MHB)

    Internal Thread:
    Minor Diameter Min = 0.4610, Max = 0.4676

    Pitch Diameter Min = 0.4768, Max = 0.4804

    Major Diameter = 0.5000

    These dimensions are from the 17th Edition of MHB (1964)
    https://www.google.com/search?q=thre...hrome&ie=UTF-8

    And I agree with the others unless you grossly overbore the muzzlebrakes you make you stand a good chance of having a conflict.

    Threading barrels is best done single point on a lathe.
    je suis charlie

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