Q regarding barrel thread repair
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  1. #1
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    Default Q regarding barrel thread repair

    Hi folks, I feel like this is a newb question, so I'm posting it in the Rookie Corner.

    My question on how to repair some issues with the thread on my SP5, and I'd appreciate advice on this. Really the question is, should I try a thread repair tool? Take it to a machinist? I'm a newbie when it comes to threaded barrels (just moved out of California last year), so I don't have any experience with it.

    Here is the backstory with some pictures:

    Last month, I picked up my new SP5, as well as a 9mm modular suppressor to go with it. My range, which is also the NFA transfer dealer, has the suppressor there. I visited the range and attempted to mount the suprressor (which has an insert reading 1/2-28), but it got stuck and would not thread onto the barrel. I took it to the gunsmith at the range, they measured the threads, and said this was not fitting their thread gauge and was not 1/2-28.

    I contacted the manufacturer, letting them know, and also sent a picture of the welds, which had some defects, and asked whether this was normal. The suppressor was backordered so I had bought it from another dealer sight unseen. The mfr said the welds were not normal and asked me to send it back for repair. They also sent me a replacement thread insert. (I would have just used 3-lug to begin with, but these were also backordered.) I tested the replacement and it went onto the barrel smoothly. The weld repair was arranged between my NFA dealer and the mfr.

    Fast forward to a month later, I went to inspect the repaired suppressor, which looks good, and tried to put it on the SP5. I thought I had replaced the thread insert prior to repair, but oops... clearly I hadn't. The suppressor got stuck when screwing on, and now the thread protector won't even screw back onto the barrel, so it looks like just trying to put the suppressor on did a slight bit of damage to the barrel threads.

    Needless to say, I'm not thrilled with the QC of this manufacturer's machining, and even less thrilled with myself for trying the same defective insert again when I thought I had replaced it. They have been very responsible and helpful for both repairs so I'm not discussing the brand here. Anyway, my question, is how should I address fixing this to avoid bungling the OEM barrel even further? Can I use a thread repair kit? Take it to a machinist? ...your input is welcome and I'm ready for the snarky comments, if necessary. Here are some pics:


    Q regarding barrel thread repair-img_2372.jpeg

    Q regarding barrel thread repair-img_2373.jpeg

    Q regarding barrel thread repair-img_2374.jpeg

    Q regarding barrel thread repair-img_2375.jpeg
    Last edited by TechOps; 04-26-2020 at 12:47 AM.

  2. #2
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    I am more than a little surprised that you were able to cut into the threads the way you clearly did. How much force were you applying? I would have thought the hammer forged barrel steel would be hard enough to resist recutting, but I guess I am wrong.

    A suppressor, like any precisely threaded tool, should need little to no pressure. If it does, something is wrong.


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    I don't have a force measurement that I could share, but definitely wasn't wrenching on it or twisting hard. And I made sure the barrel and suppressor were lined up properly so as not to cross-thread. Also don't know how hard the hammer forged barrel steel on the SP5 is, or whether it was properly heat treated, but it doesn't seem very hard.

    Thanks for your comments regarding precisely threaded tools. The suppressor in this case was not precisely threaded. Since it's the first suppressor I've ever screwed onto a barrel, I had no frame of reference regarding how much pressure is needed to screw it on. If you have any helpful suggestions on how to best remedy the situation, I'm all ears.

    Quote Originally Posted by veritas View Post
    I am more than a little surprised that you were able to cut into the threads the way you clearly did. How much force were you applying? I would have thought the hammer forged barrel steel would be hard enough to resist recutting, but I guess I am wrong.

    A suppressor, like any precisely threaded tool, should need little to no pressure. If it does, something is wrong.


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    A die can most likely be used to clean up the threads since it appears that only the first two threads are damaged. You should take it to a competent gunsmith and not attempt the repair yourself unless you have expertise in the use of dies to clean up threads (and it doesn't sound as if you do). It won't look perfect when the work is performed, but you should be able to install your thread protector and a correctly threaded suppressor.

    Second to veritas' comment about the amount of force used to install a suppressor. If it doesn't thread on easily, there's an issue that more force probably won't solve.

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    I *always* spray new threads down with Liquid Wrench (lubricating oil) before trying to thread anything. Often times in the manufacturing process, debris can be left behind in the threads that is hard to see. The capillary action of spray oil will remove such debris.
    I agree with Veritas. How did you manage that damage with just hand tightening??? Barrel steel seems soft to me if that was the case! Are you sure you didn’t jack on it with an open end wrench or (even worse) an adjustable wrench/pliers???
    You need to get it to a competent H&K smith and machinist to fix. To do it right, I’d remove the barrel and chase the threads with the barrel chucked into a lathe. Don’t send it to a cheap gunsmith hack. It could end up costing you much more. You’ve been warned!
    Last edited by CGuns1; 04-26-2020 at 05:43 AM.

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    Anyone with an adjustable 1/2-28 die and a little knowledge can fix that for you in less than 10 minutes. Using dies does take a little finesse, especially with finer threads to ensure you don't cross thread the die.

    Open the die by tightening the adjusting screw to a point where it threads over the existing threads with a little 'play'.
    Loosen the adjusting screw (to close the die) in very small increments, turning the die back past the bad threads each time
    Continue until you barely feel drag on the existing threads
    Cold blue threads

    If this makes you nervous, find someone to help. If I were a gunsmith, I'd feel guilty charging more than $5 for a such a 'repair', but who knows. In fact, I might even do it for free to gain a customer.

    Go to an HK smith and have him remove barrel and put in a lathe to retrace the threads? It's a little booger on the first two threads and there's more risk in not hitting the threads perfectly with that method. That's absolute overkill. Or a way for someone to charge $100 or more for work not needed.

    Any local members that can help him out? If OP was near me I'd do it for free.
    Last edited by esh21167; 04-26-2020 at 01:14 PM.
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    Quote Originally Posted by esh21167 View Post
    Anyone with an adjustable 1/2-28 die and a little knowledge can fix that for you in less than 10 minutes. Using dies does take a little finesse, especially with finer threads to ensure you don't cross thread the die.

    Open the die by tightening the adjusting screw to a point where it threads over the existing threads with a little 'play'.
    Loosen the adjusting screw (to close the die) in very small increments, turning the die back past the bad threads each time
    Continue until you barely feel drag on the existing threads
    Cold blue threads

    If this makes you nervous, find someone to help. If I were a gunsmith, I'd feel guilty charging more than $5 for a such a 'repair', but who knows. In fact, I might even do it for free to gain a customer.

    Go to an HK smith and have him remove barrel and put in a lathe to retrace the threads? It's a little booger on the first two threads and there's more risk in not hitting the threads perfectly with that method. That's absolute overkill. Or a way for someone to charge $100 or more for work not needed.

    Any local members that can help him out? If OP was near me I'd do it for free.
    This right here.

    Good GAWD, people. The OP didn't do anything severely wrong and nothing anyone couldn't or hasn't done themselves. No wrench required, no brute force. There are no "soft barrels" or hard this or that. And pull the barrel off an HK roller gun to correct a threaded muzzle? *sigh*

    esh21167's advice is solid, OP. It's not the end of the world and it won't cost hundreds to correct. Ignore the doom and gloom above and search out someone with the correct tool and a little time who's willing to patch it up for you. Like the above fellow with the sound mind and advice, were you closer I'd do it for free. Between bites at dinner. While chatting up the wife about the weeds in the lawn.
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    I disagree. To do it right, pull the barrel. A lot of finesse is required to run the die on the gun and making sure alignment is correct with the barrel...

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    Thanks guys for the responses, this is very helpful. I promise I didn't use a tool or yank on the thing! I was simply trying to thread it on at the range bench. Anyhow, I'll seek out a gunsmith to use a die and correct the threads. (and re-blue the area?) Most likely I'll end up using a 3-lug adaptor anyhow, so it's not a huge issue once the threads feed smoothly.

    And I appreciate the offers to do it for free, you guys are gents.

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    send @eMGunslinger a PM pretty sure he could handle this or steer you in right direction
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