Whetrock, can you give us a rule of thumb on the number of threads on a breech plug
and any variables that would change the # of threads?
Bear,
You are asking a valid question, but it’s not an easy one to answer. It’s kind of like asking how close you can get to the edge of the grand canyon without falling in. It’s best to stay behind the guard rail. I understand that "a rule of thumb" is a guardrail of sorts, but any discussion of minimums assumes certain quality in materials, consistent technique, and expert skill in the application of techniques. We don’t have that here. Yes,
variables. Variables are the problem. Barrel makers and plug makers are using various kinds of metals and different techniques in production. Builders use varying techniques as well, with varying levels of skill. And the end users “inflict” their own abuses, too, introducing even more variation in their choices regarding powders, load, and cleaning. All that variation and those variables make it tough to reasonably discuss minimums. It should also be obvious that any discussion of minimums introduces potential liability risks.
I’m not a professional gunsmith. Even if I were, I would not/could not take on the responsibility to define or even to discuss minimums in any sort of authoritative way.
But I can refer you to some published references. These do not answer your question specifically, but I think they may be useful.
To show historical continuity in the principle of bottom tapping, I’ve listed three references, from 1771, 1883, and 2014.
Regarding the length of plug, I’ve listed three references (but two of which are related). I know that many of us don’t have access to these books, so I’ve quoted excerpts.
Sprengel. “The Gunmaker and Gunstocker”, published in 1771. Translated in JHAT, June 1988, III, p 25-28) (Figures refer to a woodcut image published on page 26 of the JHAT volume. All notes here are from that translation.)“(p 26) As is well known, a breechplug [Schwanzschraube, or literally tail screw] is screwed into the breech [here unterste Mündung eines Laufs or “lower mouth of the barrel”], and therefore threads must be cut into this opening. The pointed thread tap [in] Fig.XIII “a” [starts the thread in] the hole first, and in order for this stout tap to be easily turned, a tap wrench [Windeisen, or winding iron] “cd” is attached to its square shank “c” (p28) while tapping. The same is also true for the bottoming tap [kalibermässignen Bohrer or gauged bit] Fig. XIII “e,” with which the female thread is finished out for the breechplug. The last tap therefore must be of a [constant diameter for its full length], like the breech plug. The [threads of the] breech-plug are cut with the screw plate [the word Schraubenmutter is also used here; it also signifies a nut or female thread] Fig. XIII “f”, which the bottoming tap “e” fits exactly. Therefore, the bottoming tap “E” with its accompanying screw plate “f” form a powerful cutting mechanism.”
Stelle and Harrison. The Gunsmiths Manual, first published in 1883. reading here from 2013 edition. p 166- 169.Stelle and Harrison first note that they have occasionally seen breeches tapped with a “blacksmith’s tapered tap” and plugged with an accompanying tapered plug. But they criticize that technique (as well as breeches made with a crooked thread), and pick up with the following:
“Let the workman discard all such ways of breeching guns. Let him procure a set of taps of the sizes and threads as noticed in the beginning of this article, and “stick to these sizes.” … Breeching taps should be made in pairs, one tapered a little and its mate made straight and with a full thread, so as to cut full at the bottom where the thread terminates.”
They also describe how to make a tap with a protruding “stem or projection” that will help the tap align with the bore. (They provide woodcut image, figure 36, and the image shows the tap made with bottoming threads. A similar tool, tool #1, is shown on page 64, plate 46, of Dillon’s “The Kentucky Rifle”, 1967 edition.)
From the current TOW website, downloaded June 11, 2014:http://www.trackofthewolf.com/Categories/PartDetail.aspx/1012/1/LABOR-BP “Zero clearance bottom tap: nearly all muzzle loading barrels are factory threaded for the plug. None are completely finished inside. Only the machine work is done. The final bottoming tap step must be completed by you, by hand.
A factory finished bottoming tap has one (1) thread relieved (chamfered) to allow the tap to be easily started. Make a special zero clearance bottom tap, by grinding the end of the tap perfectly square, with no chamfer. This special tap will be a bit more difficult to start, and may "chip" the front tooth, when you hit bottom. This is normal.”
Buchele, Shumway, and Alexander. Recreating the American Longrifle (here reading from the 1983 edition, p 28-30)“The depth of the threaded portion of the barrel breech, and therfore [sic] the length of the breech plug, is a matter of concern. Old barrels typically had a very coarse thread and usually the length of the thread was about ½ inch. In recent decades barrel makers often provided barrels with a thread depth of 5/8 inch, or even ¾ inch. Such threads lengths are suitable for the extra heavy barrels of bench rifles. But in recent years the makers of barrels for off-hand rifles have determined through tests that a properly fitted breech plug of ½ inch length is suitable for a barrel of almost any caliber.”
Note the words “a properly fitted breech plug”. They do not specifically mention bottom tapping, but their diagram (page 28) shows a plug with no thread relief on either end, and shows a breech that has been threaded with a bottoming tap.
Alexander. The Gunsmith of Grenville County, 2002, p 63-65.Peter Alexander (who co-authored the Buchele, Shumway and Alexander volume listed above) continues that discussion in his own book, where he repeats some of the information that was provided in _Recreating_.
“Original rifles had breeches never more than a depth of ½ inch and often less. Recent tests have proven that a depth of ½ inch is quite safe for any caliber and load, assuming the plug is properly installed.”
Alexander does not specifically mention bottom tapping, but his diagram in fig.2 show a plug with no thread relief on either end, with total length marked as ½ inches, and with the hole bottom tapped. His fig. 3 similarly shows a breech that has been bottom tapped.
Ehrig, Miller, and Dixon. The Art of Building the Pennsylvania Longrifle. (3rd printing, revised. p 22-25.“If one is cutting the threads for the breech plug alignment, keep in mind that the length of the threaded plug should be at least as long as it’s diameter. Longer if desired. This is the maximum for mechanical strength.”
They do not specifically mention bottom tapping. Their diagram (p 23) is a sketch, but it seems to show a breech that has been bottom tapped. No thread relief. It is labeled “Proper Plug Fit—No Gaps” and “Plug Length Equal to Diameter”. Their diagram on p 24 also seems to show a hole that has been bottom tapped.
I hope this is useful. I know we have spent a lot of time on this already, but I think this is an important topic that needs careful consideration.
