Should A Sealant Be Used On Breech Plug Threads?

[Zonie]

The threads used in the breech of a muzzleloading barrel and on the breech plug are regular Unified National series machine threads.

The common ones are designated by the letters UNC and UNF (Unified National Course and Unified National Fine).

None of these UN threads are gas tight.

There are clearances between the Major (largest) and Minor (smallest) diameters of the internal and external threads. In addition to these clearances the male thread is slightly smaller than the female thread and the male tooth thickness is slightly less than the female tooth space.
All of these clearances help to make the threads screw together easily.

While there are threads that will make a gas tight joint they are never used in the breaching of muzzleloading guns.
Such gas tight threads are the common Pipe Threads found on common iron/steel pipe and Dry Seal threads, also used on pipes.

These sealing threads rely on tooth thicknesses that will interfere with each other on both the sides of the tooth form and at the major and minor diameters.
To achieve this the male thread is tapered so that it becomes tighter as it is screwed into place.

The reason these self sealing threads are not used in muzzleloaders breeches is the taper of the male thread creates a tremendous amount of stress in the assembly.

The sharp pressure rise (or spike) of exploding black powder coupled with this pre-existing high stress due to the tapered threads could cause a failure.

IMO, a properly breeched barrel will have the face of the breech plug screwed tightly against the shoulder at the bottom of the threaded hole in the barrel making a gas tight seal.

If this is not done, a small amount of powder gas will escape thru the threads often resulting in a small trace of smoke around the breech plug when the gun if fired.
This usually doesn't cause a real big problem but it is somewhat disconcerting.

About the worst thing that will happen is the powder fouling will eventually plug up the clearances in the threads but it can absorb water during cleaning.

As it can also absorb oil, a shot of a good penetrating gun oil like Barricade will protect the threads.

If the builder wanted to give the threads some protection and make sure the breech plug will not freeze in place a light coating of Anti-Seize will help.

 

[jerrywh]

I use plumbers pipe dope on my breach plugs. I have used it for 50 years.

 

[Bubba45]

A light coat of the silver anti-seize is what I use . As what mentioned above the seal is made with the face of plug and bores breech.I remove the first 2 threads at face of plug down to root diameter of the thread so that they dont interfere with fitting to bores breech.

 

[ApprenticeBuilder]

I understand and use machine threads in my daily work, I also understand and use the taps and dies that make those threads in my work. I also am intimately familiar with pipe threads and dies in my daily work and their intended function.

I also know that the machine thread taps and dies are tapered to facilitate getting them started onto the material. If the taps and dies were used and the materials were cut to the full depth of those respective dies then we would be able to turn the breach plugs all the way into the breach by hand other than the crush fit of the face of the plug against the recess of the breach area, this has not yet been the case for the 8 or so barrels that I have breached.

Technically the threads of the breach plug are not designed to seal the gasses, in the real world application of the taps and dies that we use some sealing takes place.

 

[illuveatar]

I made and used a zero-clearance tap by removing the few tapered threads of a bottom tap. This allowed me to thread the breech all the way to the recessed collar. After this I was able to turn the breech plug by hand all the way in and only used a wrench to tighten at the last half flat.

How much do you normally have to "crush fit" on your barrels ?

 

[Vaino]

On some LRs I have istalled the TH liner back far enough that it "broke" into the counterbore seal and the face of the breechplug had to be "funneled". Why did I do this? The reason was to achieve better wrist architecture. Even w/ violating the plug/bbl seal, I found no evidence of gas leakage from the threads. Reason...the gases prefer going out the TH so why should the gases spiral around close fitting breech plug threads? The gases take the path of least resistance.In our "modern quest for perfection" mostly caused by lawsuits...the threaded length of the breechplugs are excessive and partly caused by lawsuits and a "CF pressure mindset". The force exerted on a breechplug face is only that of the bore dia X the pressure and w/ BP, the pressures are nowhere near that of CF cartidges. If a mishap occurs w/ a BP gun and splits the bbl, the breechplug is usually intact...anyways on the photos I've seen......Fred

 

[ApprenticeBuilder]

On some LRs I have istalled the TH liner back far enough that it "broke" into the counterbore seal and the face of the breechplug had to be "funneled". Why did I do this? The reason was to achieve better wrist architecture. Even w/ violating the plug/bbl seal, I found no evidence of gas leakage from the threads. Reason...the gases prefer going out the TH so why should the gases spiral around close fitting breech plug threads? The gases take the path of least resistance.In our "modern quest for perfection" mostly caused by lawsuits...the threaded length of the breechplugs are excessive and partly caused by lawsuits and a "CF pressure mindset". The force exerted on a breechplug face is only that of the bore dia X the pressure and w/ BP, the pressures are nowhere near that of CF cartidges. If a mishap occurs w/ a BP gun and splits the bbl, the breechplug is usually intact...anyways on the photos I've seen......Fred

Yessir I agree completely, I have also violated this so called un-violateable space. In so doing I like a bit of drag in my threads as we get to the end of the tightening sequence, an added level of protection.
As it was put to me, nothing is sacred when it comes to building, each project is an entity unto itself, and the rules are applied on a sliding scale as it were.

 

[ApprenticeBuilder]

I ground my bottoming tap out of a regular tap.


Its not a huge amount of drag that I like to see, but I am not a fan of finger tight up to the half flat, I want to put a wrench on my breach plug at the last full turn or so.

 

[Vaino]

I wonder how many bbls in use today don't have a plug face sealing off on the bbl counterbore and perform w/o gas leakage...many MLing shotgun bbls just have the threads w/o any shoulder to seal against and they work as well......Fred

 

[4570tc]

To add that there are 3 classes of threads, 1a 2a 3a, with the 3 being the tightest, and 1 being the loosest. a number 2 is the common bolt nut thread,
(just finished a chapter on this very subject) advanced machine lathe fundamentals.

 

[jerrywh]

I often turn my own breach plugs and when I do I figure a 90% fit. A normal hardware bolt and nut has a 75% v thread. As do the common taps and dies. I will use a common tap for the barrel but turn a plug with a 90% fit and face off the last few threads. This will drive some of you guys nuts but I don't worry too much about the seal on the face. Say what you will. I have taken plugs out of barrels that were 150 years old that were breached the same way and they showed very little corrosion past the threads.

 

[lonecamp]

A firm beleiver in being able to remove breech plugs,for what ever reason may arise, some type of release agent should be used.
I have found in originals pitch or tar and I know these guns had not been apart many years. Not so much to seal the threads from pressure but more to seal them from corrosion from cleaning.

 

[Jblk]

If you are going to use anything go to the hardware store and get some teflon tape used for gaspipe joints.It will be yellow instead of white in color.You could also use chalk in the threads.