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Old January 08, 2013, 19:41   #88
gunplumber
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Join Date: Jul 2000
Location: Surprise, AZ
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Quote:
Originally Posted by kotengu View Post
Three things at work here:

1. What GP is talking about is the thread deformation from tightening, which is a good thing as long as you don't tighten too far and break anything. The threads slightly deform until they more evenly make contact and spread the load. What he's doing is smart in general. I'm curious though (Mark) - are you uniforming (cutting) the threads after doing this, or just using this process to uniform and make sure you're timing right?
You know, but others might not. There are thread designations - pitch, and angle. Most are 60degree, but there is also ACME (flat) and Witworth (55degrees like on mausers). Then there is classifications of thread, referring to the tolerance of the D(maj) and D(min) Or the diameter top of thread to opposing top of thead, and diameter bottom of thread to bottom of thread. "Class A" refers to male, and "class B" to female. 1A is a loose fit, such as an AK flash hider, designed to be easily removed by hand, in the field. 2A is normal and 3A a tight tolerance. The FAL is supposed to be 1"x16tpi, 60 degree, Class 2A.

For whatever reason, I sometimes end up with receiver to barrel combinations that are "too tight". I should be able to turn the barrel by hand - presuming it is free from caked oil and dirt. Sometimes I can't. My 1"x16 bottom tap is a set size, I can't change it. But my 1"x16 die is slightly adjustable. So I first chase the receiver threads with a tap. If it is still tight, I chase the barrel threads with the die at its largest opening. Usually, the cause is a flattened thread somewhere - causing the 60 degree angle of the thread to be mushed down a bit, and although shorter than the average D(maj) is is "wider". Sometimes that isn't enough either.

So I tighten the die a bit more and chase the threads again. I'll often see one side (leading or trailing edge) getting shaved, but not the top or other side. So maybe the lead screw in threading was off by a hair? Who knows. But I'm not going to muscle it on. For whatever reason, it isn't 16 threads per inch but maybe 16.01 tpi. Or maybe the cutter was a little off, making the thread 59 or 61 degrees?

After chasing them, they screw together pretty well by hand. But if the (leading or trailing) edge of the barrel thread is now different than the receiver, I may not have every thread engaging. Does it matter? Probably not. I've heard that 3 complete threads at 60% engagement is more shear strength than the shank that is threaded. Anyway, by cranking it down a couple time, I "form" the male and female threads to each other just a little more than they were before. While it may make no practical difference, it satisfies my immense ego to know it is as good as it can be and I can stamp my mark on it.

I suspect there is some minute deformation of the threads during firing. When unbreeching a typical IMBEL barrel from the receiver stub, I need a 4' breaker bar and a 20 ton press. I hop up and stiff-arm the breaker bar, adding my svelte and debonair 225 pounds to the bar. Often I hear a "pop" and then can unscrew by hand. Other times I have to use the 14" 1/2 drive for many rotations. Since this is obviously more than the 120 ft pound target, one of two things has happened.

1. IMBEL uses a hydraulic ram and cranks that barrel on regardless of how much pressure it takes or

2. The stretching of the threads during operation deforms them enough that they no longer match the original tolerance and classification and I have to force the deformed part through the non-deformed part. Given the differential heat treat of the barrel and receiver, I assume the deformation is on the barrel side. or

3. Something else I'm not educated enough to determine yet.
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T. Mark Graham, Master Gunsmith
Arizona Response Systems, LLC

Last edited by gunplumber; January 08, 2013 at 19:50.
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