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Question fitting GM breech plug

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fpatton

32 Cal.
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Hello,

I'm working on my first kit, the TotW Kentucky flintlock pistol. I didn't have Track fit the breech plug, which I probably should have, but I have machining experience and figured it would be okay.

Peter Alexander's book says that the breech should only be 1/2" long, but the Green Mountain barrel and plug are exactly 5/8" each (to within a couple of thousandths). I decided to start by just trying to fit the plug at that length, and got it seated pretty well. However, I had to take off the first thread and a half or so to get it seated. So, if I were to cut the breech down to 1/2", I'm afraid I wouldn't end up with enough threads on the plug.

The next issue is that with the plug nicely seated, the Green Mountain logo stamp is on the top of the barrel. Not quite what I had in mind!

So the questions are, 1) is 5/8" going to cause me trouble when I try to line the touch hold up with the lock, and 2) should I think about taking the barrel down enough to put the stamp at the bottom, or leave well enough alone?

I know one option is to send the barrel back to Track and have them fit it, but I'd rather just tackle it myself if I can.

(Edit - Was just checking the thread pitch, and it if I take of 1/8", I'll still have the GM stamp at the top. So, maybe lop .090" off the barrel and the plug?)

Thanks!

Fred
 
Removing the first thread and a half is pretty normal in order to clear the imperfect threads at the bottom of the tapped hole.

It's pretty late so if my numbers don't add up, be sure and tell me. :grin:

You didn't say what the thread pitch is but I'm guessing it's 18 tpi. If that is true, one full turn will advance the thread .056.
Using this value, removing half of that value or, .028 will advance the thread 1/2 turn.

Using this value, if you remove .028 from the rear of the barrel and .028 from the face of the breech plug, the barrel will have to turn 1/2 turn further than it presently is to reseat itself against the face of the breech plug while the rear of the barrel will reseat itself against the shoulder on the breech plug.

That 1/2 turn will position the Green Mountain logo at the bottom of the barrel.

The 1/2 inch minus 1 1/2 threads to clear the imperfect threads at the bottom of the threaded hole would probably be safe in a pistol but it's up to you as to whether you really want to do this.

What it really does is to move the rear of the barrel closer to the center of the pan.

This is an aesthetic thing. The old original guns were often built this way. IMO, if an extra 1/8" doesn't make you think the rear of the barrel is too far aft of the center of the pan then just leave it as it is at 5/8".
 
Yes, sorry, forgot to put in the thread pitch. 18 tpi seems close enough based on a rough mic'ing.

The first time I measured, it looked like the 0.125" was going to be pretty close to 4 threads. I was thinking that if I cut off the 1/8" prescribed by the book, I'd be back where I started with the stamp, but cutting off 3 threads would get me close but with the stamp where I wanted it. (I have a small mill, so taking off this amount from the barrel and the plug wouldn't be much trouble.)

If a 5/8" breech isn't going to cause me trouble later, then I will go for 1/2 a thread. At least if I mess up, I have some more to take off!

Thanks!

Fred
 
I am just starting a rifle with a similar unfitted plug. If I want he snail of my lock even with the back of the barrel I have to take a little off the back of the barrel.

I bought a 5/8 18 bottom tap and that I will grind off a little of the tip to be able to tap threads closer to the breech face. I did the same with a 12 ga Colerain barrel recently.

My barrel isn't tapped very close to the breech face and has room for at least one more thread.
 
The main thing to look for is the face of the plug to seat against the shoulder in the barrel. These two dimensions can be altered in order to get the makers stamp to the bottom flat, where it needs to be. It is easy to do this with a lathe, but not too bad to do with a good file or two and some transfer color to keep track of your fit up.
 
Fred don't forget you really have three flats available for the makers mark to wind upon. Get or make a a bottoming tap. The big thing is to get a good fit at the breech face, think plug and seat in a globe valve. BJH
 
Excellent point. Gives me a bit of leeway! Except for the stamp, I was liking the fit because the shoulder and the plug surface were both within a coutlet thousandths, and I though it would be hard to duplicate. I will try cutting both down tomorrow.

I was planning to just run a mill by the end of the barrel to keep it square (I'm not confident of keeping the end of the barrel square using a file all the way), then file off the plug unti I get the depth right all around.

Fred
 
Like I said, with a 18 tooth per inch thread removing .028 will rotate the barrel half a turn to put the logo on the bottom 6:00 o'clock position.

If you just want it to be on one of the bottom 3 flats you can remove .0208 to rotate it 3 flats to the 4:30 o'clock position or .037 to rotate it to the 7:30 o'clock position.
 
When cutting the plug depths and fitting shoulders always remember to use the rule of halves when final fitting.
When fitting threads one must remember thread and shoulder compression as most threads will turn up at least a half a barrel flat after making shoulder contact.
 
Is a viable plan B to file the barrel flat until the GM stamp(s) no longer show, or would that thin the barrel too much? I suspect caliber and remaining wall thickness (as well as you desire to have the maker's mark show at all anywhere) would dictate that.
 
Okay, I think I'm done. I started by skimming 20 thou off the barrel, and about 15 off the plug. I figured that if I had issues, that would leave me two more flats to play with. After some careful filing on the plug, I got tang lined up with the 4:30 flat, and if the ink on the shoulder is telling the truth, a pretty good fit between the plug and the shoulder.

There's a very small gap between the tang and the barrel, probably 2 thou or less, but I think when I start filing, that may get worked closed. I figure I could go for the next flat and muck it up, or leave well enough alone!

Thanks for the help. Next step, barrel inletting!

Fred
 
Now that you've worked so hard to install the breech plug, don't forget to remove it before you start inletting the barrel. :rotf:

(Yes, I'm serious. The tang will just get in the way until you've got the barrel fitting properly.
Then install the breech plug and inlet the tang. :thumbsup:
 
Yep, got it! One question, though. Should the bottom surface of the tang piece be flush with the bottom of the barrel? Currently, it sticks down by quite a bit. Or is it supposed to be that way?

Fred
 
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