breech plug fit

[dlocke]

I have started to fit a breechplug in a 50-cal GM barrel and have a question. I have read everything possible regarding breech plug fitting and searched the archives.
First, the rear of the barrel is square to the sides and I measured .625" from the rear of the barrel to the inner shoulder. My breechplug also measures .625" from the bolster to the breech face. With that in mind, I applied prussian blue to the shoulder and screwed the plug in as far as I could with my fingers, then gave it a bit of a tweak with a wrench to make sure it was as far as it was going to go. The gap between the bolster and rear of the barrel measures .077", but there was no transfer of the blue dye to the brechplug. That is my issue. Does this mean that I should now start to carefully file down the breechface? Or is there something I am missing?
DJL

 

[dlocke]

Actually, I think that the problem is that there is a gap between the barrel threads and the rear shoulder of the rifled portion. When the threads run out the plug will stop turning. I know that some manufacturers will turn down the plug (i.e. remove the threads) to compensate for this, but that doing so can lead to a "quick & dirty" result and fouling can get trapped. I would guess that I need to start filing at an angle to "turn down" the threads and get a proper fit.
DJL

 

[Vaino]

Could be thread interference in the bbl because a bottoming tap wasn't properly sharpened and not allowing the plug to seat on the shoulder or the threads weren't run up to the breechplug face and the countersink in the face of the bbl isn't deep enough for the threadless portion of the breechplug. I would "blue up" the end of the threads and the unthreaded part of the plug forward of the tang and see where the interference is. Have breeched a few bbls and have run into interference at both places....Fred

 

[dlocke]

Yeah, it's definitely a thread issue. The countersink seems ok, but that assumed that the plug should keep turning after the threads reach the bottom of the countersink. That's as far as I can turn the plug by hand. The unthreaded portion does not yet enter below the countersink. How deep is that countersink supposed to be? I think it is the forward threads, near the shoulder. Everything I've read states that the face of the breechplug should mate against the shoulder where the rifling begins. My problem is getting the thing there. I might be able to force the plug in by reefing on a big wrench, but it seems to me that would cause more problems than it would solve.
The only thing I can think of now is to carefully work the forward threads down on the plug.
Kind of makes me wish I'd spent the 25 bucks to have someone else do it, but then I'd just have to do it a first time myself sometime in the future.
DJL

 

[Vaino]

I use a 12" wrench w/ 1/16" copper inserts when fitting a breechplug and lean on it pretty hard. Always use liberal amounts of lube to prevent tearing and binding. Fitting a breechplug by hand is a hard and tedious job and once the plug is "home", the work begins in aligning the bbl flat and tang. I've reground a number of taps for a true flat bottom and run it in before even starting the breeching to eliminate one trouble spot. Also one can measure the bbl ctsk and compare it to the unthreaded portion of the plug. Good luck.....Fred

 

[chuck-ia]

I had to file off the bottom 2 threads on the plug on my gm 50 cal. barrel, this was the starting point, getting the face of the plug to contact the barrel shoulder. Then filed the face of the plug to get the tang to contact the barrel, then with both touching, (tang to barrel and plug face with barrel shoulder) started to sneak up on the barrel flat I wanted. I have the GM insigna on the bottom. flinch

 

[dlocke]

Flinch, that's the situation I have and I have to do the same thing. My $64,000 question is how did you go about filing the threads? I don;t want to buy another breechplug. Unfortunately, I found some breech runout in the barrel when I put the prussian blue to it, so my GM stamp has to be either top or bottom.
DJL

 

Yes, you are missing something. The base of the threaded portion of the breech plug is never perfectly threaded and will never screw home without excessive force, no matter if the end of the breechplug contacts the end of the rifling or even if it is too short. This is more of a problem with patent breeches than with tanged breech plugs, but even with them it causes problems.

Chamfer the mouth of the threaded end of your barrel enough to allow for that, then try fitting your breech plug. Remember to re chamfer if you have to remove metal from the threaded end of the barrel.

 

[Zonie]

What you need to do (as mentioned above) is to file off the first two threads (or two and a half. You need to remove slightly more than the distance you measured between the face of the breech plug and the rear of the barrel)
.
Use a flat mill file and file these two threads down so that all that's left is a witness of the old threads root (the bottom of the threads).
If you file a little too far, or too deeply into the center body there is no harm is done, but don't get carried away. Take your time

When these two (or two and a half) threads are gone, the breech plug will screw in to shoulder on the barrel shoulder like it should.

zonie

 

[dlocke]

Sounds like a plan, Zonie. I'll mask off the threads I don't want to touch and start by taking off the first 2 threads, and then test-fit.
DJL

 

[dlocke]

Oh yeah, that did the trick. Took down the first 2 threads w/ a mill file & then smoothed everything up w/ a swiss needle file. Plug went right in until it hit the shoulder. Fortunately, it looks like it is going to fully seat on the flat that I need it to seat on (there is a little runout so the tang has to seat along the "GM" stamp flat or the one directly opposite. The tang goes about 1/4 of the way onto the "GM" flat now and I am getting good impression on the breechface. From the looks of it, once the tang is fully on the flat it will be fully seated with no gap to the rear of the barrel.
Thanks for the advice.
DJL