Breech plug length

[whitebear]

I have a friend who is a self taught modern gunsmith, muzzle loading rifle builder, and machinist.  I was talking to him the other day and he told me that the rule of thumb was that a breech plug should screw into the barrel at least as far as the width of the breech plug.  In other words a 5/8 diameter breech plug should be screwed into the breech of the barrel at least 5/8".  This was new to me anyone else ever heard of this?

[Bob Roller]

Thread length,as far as I know has always been "Maximum strength is achieved once the threades shank is screwed into its diameter".The illustration of 5/8x5/8 holds true.

Bob Roller

[Don Getz]

As Bob said above, the "maximum" strength, but do you really need it?    How many persons on this forum have ever looked
at an "OLD" breechplug?  When was the last time you heard of a breechplug blowing out of a barrel?  If you would have been
building guns in the 1970's and used a Paris barrel, you would have found a breech plug done with a 5/8 x 11, or 3/4 x 10,
both would have been 1/2" deep, and at the time were considered the top of the line barrels.  When we moved their equipment
to Beavertown, we continued with those coarse threads for a while.  We had to chase those coarse threads on  a lathe,  what
a bummer.  We eventually went to a threading head on an old rotary head lathe.  We needed a quick change gear box to
change speeds.   At the time we had a friend who was a good tool and die man, worked a few hours in the mornings for us.
He came up with a "Volvo" transmission, complete with a gear shift....it not only worked great, but made for an interesting
piece of equipment.............Don

[LRB]

  I've seen pics of 18th c. BPs that were measured at about 3/8" in length, and lined up at just finger tight.

[Acer Saccharum]

I've examined a few antique barrels where the plug rattled around in the breech. Easy to clean, I suppose. 

[jerrywh]

 I have four friends that are engineers. According to them a plug or a bolt reaches maximum strength when there are full threads engaged that equal I/2 of the bolts diameter.  That is full threads. On a barrel with properly installed breach plug The barrel is at least four times as likely to fail than the plug is.  Three of my friends are aircraft engineers for Boing or were.

[D. Taylor Sapergia]

I took the plugs out of a Staudenmayer 14 gauge flint shotgun barrel set, and they were simply bottomed at the ends of their threads with no attempt to make a face.  They also bottomed on the ends of the barrels, of course, but the point is, these breeches are 3/8" long and are in perfect condition, still showing case colours internally and shiny metal in the threads.  Mind you, a shotgun develops less breech pressure than a small bored rifle....

[Dave Dolliver]

The standard saying, "Screw length in the nut portion should equal the bolt diameter," is to develop the full TENSION capability of the bolt.  That said, consider that the threaded portion of the breech plug is NOT being loaded in tension; it is a PLUG  being loaded in COMPRESSION. 

The load is roughly the chamber pressure times the area of the bore (if the plug is seated properly against the barrel shoulder) and this is significantly less than the tension strength of the threaded portion.

I have searched for a proper chamber pressure to use for black powder muzzle loaders and the best one I have come up with is 20,000 psi.   I'm not real happy with it and would appreciate any info on a better value.

Even if the entire diameter of the plug were exposed to the chamber pressure the stress to be transferred  to the threads would be on the order of half the tension strength of the threaded portion that would require a full diameter length of thread.

I feel the 1/2" length of breech plug thread that many of us have adopted as standard practice is adequate for our applications if installed properly.

Yes, I was one of the ones Jerry asked about this question.

Dave Dolliver

[Dphariss]

  I've seen pics of 18th c. BPs that were measured at about 3/8" in length, and lined up at just finger tight.

How tight they are now may not be how they were then, a little corrosion can change the fit. However, there were incompetents in the 18th c as well.

There is little need for a breech much over 1/2" long when the breech thrust is calculated. It takes a LOT of pressure to pull a 5/8" bolt in two. Far more than can be applied to it with a charge of black powder.

But its better to err in having a little too much than a little too little and the quality of the breeching job can effect the effective breech thrust contained by the threaded joint. Another factor is the wall thickness at the threaded area of the breech. If this is too thin then it can be a failure point rather than the threads failing. Thus in barrels under 1" across the flats an 11/16 or 5/8 thread size is preferred for the larger bore sizes used in a barrel this size.

Dan

[huntinguy]

I have four friends that are engineers. According to them a plug or a bolt reaches maximum strength when there are full threads engaged that equal I/2 of the bolts diameter.  That is full threads. On a barrel with properly installed breach plug The barrel is at least four times as likely to fail than the plug is.  Three of my friends are aircraft engineers for Boing or were.

Keep in mind that threads are generally made at  70% engagement. Darn theoretical engineers.
Also, on stressed loads (lifting) for most steels it is  1 x diameter for thread engagement and for non ferous materials it is  4 x diameter minimum. Both at  70% engagement.
But this is really an interesting topic. I have wondered about this many times. Thanks for starting it

[WKevinD]

I've wondered about this as it relates to lock placement. On a number of original 18th cent. pcs. the rear lip of the pan aligned with the breech end of the barrel.
I havn't had the chance to disassemble any to see if this was accomplished by notching the breech face or if it is a shorter plug? 

[Acer Saccharum]

I've wondered about this as it relates to lock placement. On a number of original 18th cent. pcs. the rear lip of the pan aligned with the breech end of the barrel.
I havn't had the chance to disassemble any to see if this was accomplished by notching the breech face or if it is a shorter plug? 

Cruise a museum or the RCA books, and SOME guns have the lock fence lined up with the back of the barrel. Many do not.

I don't bother with that concern. The big issue that can become problematic is when the lock is so far forward, that the top lock bolt is going thru the back of the barrel!  This also leaves little wood on the opposite side for the head of the lockbolt to be supported.

[jerrywh]

 Dave Dolliver and I have been friends for quite some time. He has made more rifles than anybody living that I know. He is even older than I am but still has all his marbles. He was a Boing engineer and a person I am proud to know.

[whitebear]

Thanks for the information everyone, keep it coming I am learning a lot and seems that some others are also.