Breech plug Help

[bbhf]

Help needed on the true length of my breech plug thread length: Barrel: Green mt 7/8- 50cal: plug is standard machined type with flared tang, with the gap between the tang end and the threads: 5/8-18: large siler flintlock: i breached the barrel with the plug as issued: then decided to shorten the length to 1/2in as per the books that i have, in order to try to get the fence lined up with the barrels end: here is my dilemma: the books say shorten the barrel and plug to 1/2in thread: if i shorten the barrels thread to 1/2in i will wind up having less then 1/2in threads on the plug due to the fact that the plug has that gap behind the threads and the tang face: does the reduced length of the plug threads really make a difference in the safety factor? please advise as to how to proceed: i hope i have explained this accurately: skip   Post Script: is there any reason why i can't use a 5/8-18 bolt and weld a tang on? i would then get the true half inch thread on the plug:   thanks       

[Bentflint]

If you cut the barrel threads to 1/2" and the plug threads to 1/2" why would you have a gap at either end. I would be thinking more about getting the stampings on one of the bottom three flats.

Removing 1/8" is 2.25 threads, meaning your tang will clock a 1/4 turn past where it is now or on the lock side of the barrel.

Bruce Everhart

[Scott Bumpus]

The gap behind the threads is the clearence cut  for the threading tool to stop in. 1/2" of threads should be plenty strong.  you will have to file down the lead thread to get it to goall the way in.  the tap is tapered and when cutting the threads it will not cut the bottom thread to full depth.  hope this helps

[bbhf]

Thanks for the replies: Bruce, Scott: is right on the thread end: the gap is between the tang face and where the plugs threads begin: can't alter this: if i want to have the plug seat in the bore and tight against the tang and barrel the actual thread length on the plug will less than 1/2in: i will have to file down the plugs threaded end to get it to seat:  granted it won't be short a lot but still less then 1/2in: am i being to fussy? i am still thinking about the bolt idea: no gaps:  thanks again for the replies: 

[Dave B]

You are describing one of the things I don't like about some plugs. The turning of the pug at the joining of the tang and lug leaving about a thread and a half gap before the face of the tang that butts up against the breach face of the barrel. I believe that as long as you have five solid threads engaging the barrel threads you will have no problems. If you feel unsure about this breach it up and do a proof load to check it by double patched ball and 200 gr of 2ff powder.

[California Kid]

Use a bottom tap in the barrel. You can grind the taper off a regular plug tap. I generally do this with GM barrels. Will give you a little more depth. Look around you may be able to find a plug without so much of a gap. They do that cause their easier to make.

[Acer Saccharum]

Rather than drive yourself crazy with the rear fence having to line up at the back end of the barrel, take a look in some gun books. A couple of books: Rifles of Colonial America, Thoughts on the Kentucky Rifle in its Golden Age, etc. You will see many, many rifles where the fence does not line up with the back of the barrel. This is a modern myth that the fence has to line up with the breech end. If it works out, great. If it doesn't, realize that builders 200 yrs ago didn't get the thing to line up either.

[California Kid]

Acer has a  Good point. I wouldn't be overly concerned with getting the fence and breech lined up. Get it as best you can with what you have to work with. I'd worry more about a good fit and orientation to the flats.

[Mark Elliott]

It is standard to have the breech plug 1/2" long.  As you have noticed, this almost never means 1/2" of full threads.   Usually, with a 1/2" plug, you get about 3/8" of full threads and that is fine as long as the plug is fit properly.    As your barrel is currently breeched, it probably has 1/2" of full threads because that is the modern standard.   If you are uncomfortable with less threads and you can still put your fence within 1/8" of the breech,  just leave well enough alone.  

The problem I have is that I usually can't put the fence back to the breach even with a 1/2" plug.  That is because if you use a 3/8" vent liner, your touch hole will be 3/16" in front of the breech plug face.   Some people will put the liner into the breech plug.   I will not as that impinges even more on that measly 3/8" of threads and provides even greater opportunities for corrosion to set in.  

If you can get your fence within 1/8" of the breech without significantly shortening your breech plug, be happy with that.   If you look at a lot of originals,  you will notice, as Acer pointed out,  the original makers didn't care much about the pan fence ending up against the breech.  They didn't even care to fill the gap between the fence and the wood at the breech.   At least I leave a little piece of wood there.  The original builders would just chisel it all out the easiest/fastest way possible.    Most of their customers didn't know any better or care.  

Mark

[smylee grouch]

In my experience the threads of a regular fine thread bolt are some times not the same as those of the breech plugs. If you hold both of them together you might notice the breech plug threads come up more and the modern bolts might be more rounded over. I dont know if this would significantly affect safty issues or not but might be something to consider. Maybe some of our other more experienced group could elaborate more on this.    Gary

[bbhf]

Thanks to all who have given me this great advice: i have all the shumway books and have noticed the variation in the lock placement: i was trying to get close: after a good day out in the shop i have the barrel with 1/2in to the shoulder: i already made the tap with the ground shoulder: i did a sample plug using a grade 8 5/8-18 bolt: i got it to seat, and filed flats for a wrench and top flat for a tang to be welded on: i know for sure i have a full 1/2in of threads: i may use this plug: it was satisfying to make it: i should mention this is my first rifle build, so i am cautious: as to the bolt shoulder i had to do a lot !!!!! of filing and emery cloth to get it even on the end: it resembles the store bought plug end: as a blacksmith who studies and practices 18th century work, i think about the old gunsmiths and how they cut threads with a jamb plate and/or a file: did they use this method on the breech plugs or did they do something else?  skip

[Long John]

bb,

With properly sized and cut threads 3 full threads give you approximately 90% of the tensile strengh of the same structure made of solid stock.  With a 5/8-18 thread, 3 full threads take up 3/18ths of an inch or about 0.1677 inches.  That's a lot less than 1/2 inch!  So let's err on the safe side and decide that we want twice the thread length that will give us sufficient strength.  Two times .1677 equals 0.333 inches.  So, the rule of thumb of 1/2 inch of breech plug thread length is MORE than enough.  A single thread more or less will not make a significant difference.

In today's world of starved, underfed attorneys barrel makers are legitimately concerned over allegations of negligence in the manufacture of their products.  So they err on the safe side.

Take Acer's observations seriously; the old time makers did not line up the lock pan fence with the breech.  English makers seemed to do that but the locks they were using had elongated pan fences.  I went through both volumes of RCA a couple of years ago and surveyed just this issue.  About 55% of the rifles the breeche was behind the pan fence, about 35% of the rifles were lined up and the remainder the photos were too muddy to determine.

Best Regards,

JMC

[bbhf]

JMC: thanks: i have looked at the RCA also and noticed that: i think i will use my bolt and weld on a tang, since i filed the barrel down to 1/2in thread length: i will have the 1/2in thread length on the plug:  the style i am building is Andrew Figthorn: skip