Muzzle thread out of spec, how to resolve?

relayer1989

relayer1989

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I have a 1/2x28 muzzle thread which is out of spec from the factory and I am wondering what the best way to resolve is?

The flash hider I want to use fits on other guns, so I do not believe it to be an issue with the flash hider.
Another muzzle device barely fit on the barrel in question, so it may be an issue of the tolerances of the barrel and flash hider working against me.

Should I:
  • Muscle on the flash hider?
  • Try taking a 1/2x28 die to correct the threads on the muzzle?
Sending back for warranty is not an option at this point.

Thanks!
 
I guess it depends what about the thread is out of spec. If it's a diameter issue, running a die over it would probably help. If it's a thread pitch/lead issue, running a die over it would likely make things worse. Do you have (or have access to) a thread pitch gauge? They're cheaper than dirt if you wanted to pick one up (Like less than 20 bucks on Amazon) to see where you're at.
 
Scotty454 said:
I guess it depends what about the thread is out of spec. If it's a diameter issue, running a die over it would probably help. If it's a thread pitch/lead issue, running a die over it would likely make things worse. Do you have (or have access to) a thread pitch gauge? They're cheaper than dirt if you wanted to pick one up (Like less than 20 bucks on Amazon) to see where you're at.
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Thanks for the suggestion.

I'll pick up a thread pitch gauge to see whats going on. If a diameter issue, would that be the inner diameter of the thread thats an issue?
 
relayer1989 said:
Thanks for the suggestion.

I'll pick up a thread pitch gauge to see whats going on. If a diameter issue, would that be the inner diameter of the thread thats an issue?
Click to expand...
Could be either. OD too big, or threads not cut deep enough. There's a lot you can screw up on a thread that will cause it to not fit. They could have used the wrong insert if they single point cut it and the form is wrong. But if the pitch is right, running a die over it will likely solve pretty much any other problem.
 
So your barrel is actually metric and this fits the barrel OK - you are now trying to put an imperial 1/2-28 flash hider on the other end?
 
relayer1989 said:
I have a 1/2x28 muzzle thread which is out of spec from the factory and I am wondering what the best way to resolve is?

The flash hider I want to use fits on other guns, so I do not believe it to be an issue with the flash hider.
Another muzzle device barely fit on the barrel in question, so it may be an issue of the tolerances of the barrel and flash hider working against me.

Should I:
  • Muscle on the flash hider?
  • Try taking a 1/2x28 die to correct the threads on the muzzle?
Sending back for warranty is not an option at this point.

Thanks!
Click to expand...
depends how bad it is. you might be able to chase the threads out with a die - simple enough job if the threads are just damaged. If they're way out (not sure how this could happen), maybe try measuring them to see if they were just threaded metric, but close enough for you to get a couple of threads in before it binds? maybe there's nothing wrong with them at all - you just have the wrong muzzle device. Just a thought.

Good luck.
 
relayer1989 said:
Correct. With the PS90 this adapter serves as the thing that holds the barrel in place since neither are connected to the receiver.
Click to expand...
As has been mentioned check with a thread gauge. I've bought a few CMMG parts years ago for AR builds and their quality seemed good. Might be as simple as chasing the threads. Ramming the flash hider onto it could just make it worse.
 
Reach out to CMMG, this particular case is extra confusing because M12x1LH and 1/2-28" are very close dimensionally. If they had a QC problem they are better knowing than not. Here's the class 3 spec for 1/2-28

class 3 specs for 1/2-28 UNEF.

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
Click to expand...
 
A die should do it all, the barrel was probably just done at the end of the run with a worn insert. Even with threads at a min/max, they will still work.

A wrong-pitch fastener will only ever complete one turn before jamming.
 
Jahnj0584 said:
A die should do it all, the barrel was probably just done at the end of the run with a worn insert. Even with threads at a min/max, they will still work.

A wrong-pitch fastener will only ever complete one turn before jamming.
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Not always. 28TPI is .036"/rev, 1mm pitch is .039"/rev. I've seen guys thread m6x1mm screws all the way into 1/4"-28 holes. Sometimes the imperial and metric pitch are super close.
 
Paul_T said:
Ramming the flash hider onto it could just make it worse.
Click to expand...
That is virtually guaranteed.


Ex3dy said:
Reach out to CMMG, this particular case is extra confusing because M12x1LH and 1/2-28" are very close dimensionally. If they had a QC problem they are better knowing than not. Here's the class 3 spec for 1/2-28
Click to expand...
The chances of a US company using a metric thread accidentally is pretty low.


relayer1989 said:
I've ordered a pitch gauge and die+handle, we will see how it measures.
Click to expand...
Dies are a little tricky to use. They can be difficult to get on straight. Be careful with the die until it is on to the thread properly. Its pretty easy to damage threads with a slightly cocked die.


Jahnj0584 said:
A wrong-pitch fastener will only ever complete one turn before jamming.
Click to expand...
Not true, depending on the mismatch of the pitch it could turn a couple of rotations. The giveaway will be that a mismatched thread will lock up pretty suddenly, rather than slowly tighten up.
 
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