breech plug tolerances

[mattybock]

what would be the acceptable tolerances for the fit of a breech plug to the barrel? How tight is too tight, and how loose is too loose?
I figure tighter is better than saying hello to hot gas.

 

[54ball]

The short answer is no tolerance at all.

The front of the plug should make a gas tight seal on the shoulder in front of the threads. This is revealed by a clean ring where your inletting compound is squeezed away due to metal to metal contact.

 

[44-henry]

If you are asking for thread fit I shoot for a no wobble fit that I can screw into the barrel most of the way by hand. I expect to have to use a wrench for the last few turns to finish it up. As was mentioned previously it should fit the shoulder perfectly with full contact.

 

[mattybock]

here's a technical bit. If I had any scrap to experiment or a vernier caliper to measure with I'd do so, but that is not the case.

If you know, how big is the hole in the breech, and how big is the diameter of the plug in thousands of an inch?

I know it has to be snug, but I don;t want to have to rent an 800 pound gorilla to screw them on.

 

[zimmerstutzen]

Found out the hard way early on. Marked a breech plug and removed it, (to switch from perc to flint.) When putting the new breech plug in I had the barrel clamped tight to the work bench, a big thing with solid oak legs and a top made from an 8 ft section of bowling alley. I had carefully measured the breech plug threads and depth and figured I had it to just a couple thousandths over size and that with a little elbow grease, the parts would mash into place. I had a 2 and a half foot monkey wrench on the square TC breech plug wrench> and only had a small piece to go, perhaps an eighth of a turn, and flipped the whole work bench over. (What a mess)

Somethings we learn by mistakes.

 

[mattybock]

did you experience any problems in the firing? No leaks, wobbliness or such?
If not, .002 inches it shall be.

 

[Vaino]

The breechplugs that I've installed tighten up approx. 3/4 to 1/2 a bbl flat from "home" which tells me that the threads on the end of the plug are interfering w/ the incomplete threads at the ctbre ledge. So, besides the "kiss off" on the ledge w/ the plug face, the incomplete threads at the ledge are also sealing. In effect, there's 2 seals.....Fred

 

[jdkerstetter]

What Fred says. I will add for clarification that it is not the threads along the length of the plug that make the gas free fit but the crush fit at the face of the plug and the end of the breech.

Many early barrels had a breech plug that was the same diameter as the bore but this practice hs been replaced by plugs that are over breech size so that the plug seals against the breech face and does not rely in the threads. The modern way is considered stronger and thus safer.

Enjoy, J.D.

P.S. Thanks for letting me out of the box Mattybock

 

[zimmerstutzen]

It depends a bit on the interior shoulder of the parts. If drilled out, the shoulder in the barrel will have the same angle as the drill, some drills are a 60 degree angle others have other angles. At the end of the breech plug threads, the angle should be close to a match.

If the barrel breech cavity was turned on a lathe, it is likely a square shoulder. the end of the breech plug threads should have a nearly but not quite square edge so the breech shoulder is thrust up against the interior barrel shoulder with enough "mashing" to seal the bore from the threads.

In automotive parts, gaskets do the sealing. For a muzzleloader, the parts need to match better so the metal to metal seals the shoulders together from fouling and gasses getting back to the threads.

 

[ohio ramrod]

I custom make my breech plugs and use a thread micrometer when chasing the threads and make the pitch diameter .001 less than the taps pitch diameter. But as stated the seal should be on the face not along the threads as in a tapered pipe thread. :hmm:

 

[Vaino]

Have used Colerain .54 cal bbls and the breech threads were 5/8-18 and seeing the breechplugs had chamfers on the end of the threads, I doubt that the plug faces sealed at all. Yet when screwing in, resistance was evident, so the "seal" was due to the crushed threads on the end of the plug. In this cal., Rice uses 3/4-16 breech plugs which do yield a "sealable" shoulder. Have to say that I didn't see any blowby on the Colerain bbls, so sealing solely w/ crushed threads does work. Fred

 

[Pigslayer]

The breechplugs that I've installed tighten up approx. 3/4 to 1/2 a bbl flat from "home" which tells me that the threads on the end of the plug are interfering w/ the incomplete threads at the ctbre ledge. So, besides the "kiss off" on the ledge w/ the plug face, the incomplete threads at the ledge are also sealing. In effect, there's 2 seals.....Fred

I am in complete agreement. I have on two occassions had to cut a little off the face of the breech plug . . . not that I wanted to. Then once I removed enough, it was all file work to get it timed right. I like the breech plug to fit tight or at least really snug. If it were "loose" I'd have to go all the way around again. I always use a little blue Locktite on the threads. So much for leakage.