Breach Plug

[JCurtiss]

So, being new to flintlocks I just learned that the breach plug when tightened into the barrel must contact two (2) shoulders at the same time. One shoulder is in the base of the barrel where the breach plug threads stop and the rifling starts.  The other shoulder is simply the end of the barrel.

The purpose, it seems, is to achieve two (2) metal-to-metal seals - one at each of the shoulders mentioned above, which seems difficult to achieve.

However, I've read that if the touch/vent/flash hole intersects the breach plug it's no big deal, merely grind a groove in the plug to allow the touch hole to communicate with the powder charge in the barrel.  But in doing so, you lose one of the metal-to-metal seals. So presumably one (1) rather than two (2) metal-to-metal seals in acceptable?  

Also, a face seal is not a very efficient seal for this application and trying to achieve two (2) of them simultaneously is beyond good engineering practice.  Has anyone ever explored a single tapered metal-to-metal seal?

Perhaps a  tapered seal is just too nontraditional for this application?

Am I asking too many questions?

Jason

 

[JTR]

There's only one sealing surface, where the plug face meets the rifling.
Where the tang meets the end of the barrel is just a junction for a smooth transition from tang to barrel.

If you don't care to try it, just buy a barrel with the plug/tang already installed. If you like, you can remove the tang to check the seal, but it seems the better barrel makers get it spot on.

John

[Dave R]

I realize many originals and contemporary builders have positioned the touch hole back too far and had to notch out the breech plug! Many builders say that this increases velocity which it may HOWEVER in theory this is good but when cleaning in between shots black powder residue gets forced into the groove in the breech plug and reduces the chance of  main powder charge igniting!! AVOID HAVING TO NOTCH THE BREACH PLUG AT ALL COSTS!! MEASURE AND RE MEASURE THE LAYOUT OF THE GUN DURING CONSTRUCTION !!

[Natureboy]

  My flinter had a touch hole that was too far back for good ignition, so I had a White Lightning liner installed, forward of the original touch hole and centered over the pan.  In do so, the drill created a notch in the plug, and that worried me a bit.  I agree that it's a bit difficult to keep it clean without removing the barrel and flushing water in and out.  Should I assume that it's not dangerous?  I suppose I could install a new breech plug, but that scares me, too.

[JCurtiss]

Thank you for the responses.

So if I've interpreted what I've read here correctly, the primary seal is between the plug face and the rifling face. Notably a secondary seal exists between the end of the barrel and the tang surface on the plug. However, this secondary seal becomes the  only seal if the plug is notched to accommodate a touch hole that  is drilled too far back on the barrel. 

[Acer Saccharum]

If your plug is larger than the bore, you can get a shoulder to shoulder seal. It's nice to have, because it is so much easier to clean. Ideally, your touch hole liner will be ahead of the plug face.

The real seal is the labyrinth formed by the threads. If your plug is just thread depth larger than bore size, there is no shoulder. This is OK as long as your thread fit is good. Old taps and threading tools produced tapered plug and breech threads.

The tang shoulders are not a sealing surface at all.

If your touchhole cuts into the breech face, it's not ideal, but it's not unsafe as long as there is plenty of plug length.

I hope that helps.

[Bob Roller]

If your plug is larger than the bore, you can get a shoulder to shoulder seal. It's nice to have, because it is so much easier to clean. Ideally, your touch hole liner will be ahead of the plug face.

The real seal is the labyrinth formed by the threads. If your plug is just thread depth larger than bore size, there is no shoulder. This is OK as long as your thread fit is good. Old taps and threading tools produced tapered plug and breech threads.

The tang shoulders are not a sealing surface at all.

If your touchhole cuts into the breech face, it's not ideal, but it's not unsafe as long as there is plenty of plug length.

I hope that helps.

Acer,
From the view of a long time machinist I say you could not have said more or made it any plainer.
I thought about answering this but was tired from a day in the shop.

Bob Roller

[David Rase]

If by "single tapered metal-to-metal seal" you mean cutting the bottom of the breech seating surface at an angle (bevel) with an matching bevel around the OD of the threaded portion of the breech plug, Getz has been doing that for years.  I think they drill the breech using a standard 118 or 135 degree drill point and then tap the treads.
David 

[Acer Saccharum]

Your tap has a beveled lead, make your plug to match the bevel of the tap. It will fit right up tight.

It's not practical to have a tap with a square cut end, as the teeth break off with very little pressure. A tap's gotta have a beveled lead.

[Herb]

There is a tutorial on fitting a breech plug.  Go to Tutorials, Metal Shaping, and the third Herb post is "Fitting a Hawken Breech Plug by Hand". 

[David Rase]

It's not practical to have a tap with a square cut end, as the teeth break off with very little pressure. A tap's gotta have a beveled lead.

You are correct, but that has not stopped me from grinding them square, over and over and over, you get the point! 
David

[Turtle]

 I seat the breechplug threads coated  with blue locktite. I have pulled breechplugs(easily) after 10 years and no gasses have gotten past the first 2 threads max if that.  I once worked for days to get a perfect seal in both spots. Life is much easier now.
                                                   Turtle

[JCurtiss]

I seat the breechplug threads coated  with blue locktite. I have pulled breechplugs(easily) after 10 years and no gasses have gotten past the first 2 threads max if that.  I once worked for days to get a perfect seal in both spots. Life is much easier now.
                                                   Turtle

Notwithstanding that blue Loctite is a "mild" thread locker, I assume you had to apply heat and some muscle behind a 15-inch adjustable wrench to remove the breach plug? 

On second thought, the blue Loctite requires only a nominal amount of torque to makeup the breach plug because you're no longer having to rely on a compression-fit metal-to-metal seal. So perhaps the necessary breakout torque was not too substantial after all?

What you're doing with the Loctite is creating a gas-tight seal and keeping the un-burned powder residue from packing-off in what Bob calls the thread "labyrinth". Notably a straight thread, which most all modern breach plugs and barrels seem to have, is useless for sealing unless a thread locker or other sealant is used.

[Turtle]

 No, the plug comes out relatively easily with no heat.  Blue is low strength locktite sealer and acts as a antisieze also-win win.
                                                     Turtle

[Long John]

As Acer said, the ideal situation is to have a plug-on-internal-shoulder fit of the breach plug.  This is ideal for 2 reasons:  it is easier to clean and it prevents the ingress of fouling into the space between the male and female threads where corrosion can occur.

If you don't have a face-on-internal-shoulder fit then the threads will serve as a seal of sorts because the length of the thread exceeds the MESG for the burning powder gases.  MESG = Maximum Experimental Safe Gap; which is the maximum gap over length ratio to achieve quenching of a deflagration.  Even the old-time threads exceeded the MESG, although I doubt that the old-timers knew that's why their breech plugs didn't spurt flame and hot gas.

When I fit a plug I don't worry about tapping all the way to the shoulder,  I file the first thread on the plug completely off so I have a nice plug-on-internal-shoulder fit.  For 60 degree threads, as long as there are 5 complete threads the threads are as strong as the male shaft in tension.  I then polish the face of the plug down to 1500 grit.  I apply filler to the threads and turn it in.  If (usually since) there is a small gap between the breech plug shoulder and the end of the barrel I just give the gap a stout rap with my hammer.  The metal is upset and fills the gap.  A few strokes of the file finishes the surface and we are done!

Best Regards,

John Cholin

[T*O*F]

It's not practical to have a tap with a square cut end, as the teeth break off with very little pressure. A tap's gotta have a beveled lead.

Actually, it is practical, but maybe some folks only have one tap.  I tap with a regular tap and then follow up with a bottoming tap.  Then I use a flat ground one to clean the taper up.  I do this a little at a time, working the tap back and forth until I hit bottom and have a flat shelf.  I have never broken a flat ground tap.

People come from a variety of backgrounds, most of which are not tool related.  With over 4 decades of experience as a mechanic, I learned early on how to judge the strength of the materials I am working with.  It is much easier to do it right than wasting valuable time extracting broken bolts, taps, etc.  This only comes with experience.

the ideal situation is to have a plug-on-internal-shoulder fit of the breach plug

One of the problems is that no one is taking the time these days to make good quality plugs for sale such as the one pictured.  You will notice that the end of the plug is turned to eliminate the first thread, and also that it is threaded completely to the bolster which negates have to countersink the end of your barrel to accommodate those last missing threads.  You can solve this problem a bit by turning your die over so the flat side does the cutting and clean those last threads up a bit.  Of course, Acer will say that you will break those threads off your die.  Right?

[Acer Saccharum]

I'm not going to argue with you, TOF.

[T*O*F]

I'm not going to argue with you, TOF.

Why would you even think you have to.  Arguing is pointless, especially when one loses one's temper in the process.  However, spirited debate is something I love and seek to engage in.  Everyone learns something in the process.  Sadly, in today's society, few are unable to distinguish between the two.

[Dr. Tim-Boone]

It is sometimes difficult to tell the two apart in print on a computer screen........ we can't see the twinkle in your eye!! 

[gibster]

For 60 degree threads, as long as there are 5 complete threads the threads are as strong as the male shaft in tension.

Long John - am I reading this correctly that the breech plug only needs 5-complete threads to have a strong/safe breech plug fit? The barrels I have used, along with the breech plugs have had about 10-threads.  Is this just a CYA thing?  I know that the originals always seem to have either the touch hole or the drum much closer to the breech end of the barrel than I see contemporary rifles having.  I have often wondered about the breech plug depth and how short of a plug would still be safe.

[JCurtiss]

Thank you all for the lively and informative discussion on breach plugs and their sealing mechanism. And true to forum behavior, there are diverse and relatively passionate ideas about the quantity and quality of seals between the breach plug and barrel.

I now feel that I can make an informed decision regarding the installation of my own breach plug when that time comes. I ordered an Andrew Verner gun kit recently and hope to receive sometime next week and certainly look forward to working on the breach-plug-to-barrel interface as well as the rest of the kit.   

Best,

Jason

[T*O*F]

It is sometimes difficult to tell the two apart in print on a computer screen

Tim,
That's why I came up with my philosophy of "Measured Indifference" when dealing with internet posting.  Whatever is said isn't worth arguing or getting upset over.  Sometimes just stating the obvious precipitates that.

........ we can't see the twinkle in your eye!!

Yeah, but having known me longer than most others here, you know it's there and I will always play the Devil's Advocate.

[rfd]

i have a relatively new smoothbore that has an unlined touch hole that intersects 2 threads of the breech plug.  a small groove was filed into the plug face to route the pan sparks to the main charge.  this became an issue, with unreliable firings because that plug face groove was simply too shallow to allow positive touch hole cleaning.  

too shallow and short ...



i pulled the breech plug and used a dremel to open that channel, replaced the plug with anti-seize lube, and ignition became 100% reliable.

just right ...

[Acer Saccharum]

I'd never say 'five threads is all you need' for a breech plug. Coarse threads are stronger than fine threads, because there is more metal engaged.

 I know there were guns made with short plugs two hundred years ago. Some of those rattle in the breech, and can be unscrewed with two fingers.

I don't like to make a short plug, so I usually use what the barrel manufacturer makes.

[Dphariss]

The threads only clamp the plug against the shoulder and give the strength needed to contain the explosion. Modern threads have enough tolerance to allow gas and fouling back into the threads 3-4 threads if the breeching allows. Either a gap of a groove filed in the brech face for the vent/liner. If the plug is properly fit there is no gas leakage into the threads.
Nor were all original plugs tapered. But when the tap makes the screw plate or the screw plate makes the tap then there is a close thread fit.
If the propellant is BP fouling in hidden areas is not a huge issue. But if the powder contains chorates which produce what is basically table salt when fired this something else. This stuff can dissolve steel with no apparent rust and will so so under an oil film. So having it trapped in breech threads or any where else is bad.
Once upon a time I had a vent liner gas cut the threads due to loose fits. A nipple without a seat at round section above the threads may well do the same thing.

Dan