4 Bore Breech Plug Threads?

[AFriendOfLife]

Hey all. I am in the process of making a 4 Bore blunderbuss barrel and I am not sure what to thread the breech too.

The bore is 1", and I plan to make the threads 1 1/8-16 to minimize the risk of corrosion getting into the threads.

I am going to make my own plug, and I am turning the threads on a lathe as they are so large.

My problem is that for the actual breech thread, I either need an internal thread relief since it comes up to a shoulder, or I will just thread to a point and cut a couple of threads off the breech plug to fit the minor diameter of the female thread if that makes sense.

That would make my threaded breech .625 deep to have .5 inches of thread.

But then on my breech plug I need a thread relief so I can thread all the way up to the shoulder of the tang. That would make my threads only .375 inches long, which sounds scary for a 4 bore.

Should I do a smaller 1 1/16-16 thread so the minor diameter is the same? Then I don't need a thread relief gap.
I'm just worried about getting fouling into any possible exposed threads.


Aha... Did I explain this well enough?...

 

[Phil Coffins]

Finer threads are stronger, if your cutting threads on a lathe you can make them what ever pitch you like. On my 1 3/4” cannon I made them 2” X 20 tpi .

 

[waksupi]

Rule of thumb, the breech plug should be a length that is at least as large as the bore size.

 

[nhmoose]

Needed info is the outside Dia of the barrel as the breech threads are also dependent on that, I would rather see bigger threads as in 1 3/16 by 16 if you can get a tap, or if the barrel is big enough 1 1/4 X 16 would be good.

Fouling getting in the threads is moot when you fit the plug properly flat face of plug mated to flat face of bore.