barrel vent repair

[bob in the woods]

The vent on my 4 bore needs repair. I have what I need to drill and install a 1/4-20 plug but wondered if a 3/8-32 would be better considering the bore /barrel size ?

[Bob Roller]

I would stay with the 1/4x20 or maybe the 1/4x28 and then IF needed the 3/8x32. How thick is the barrel at the breech on that big bore gun.
Bob Roller

[bob in the woods]

Thank you for the reply, Bob.  I just checked..the breach is 1 and 3/8ths inches across the flats

[smylee grouch]

Maybe the length of the flash channel might dictate which size to use. 

[Bob Roller]

Maybe the length of the flash channel might dictate which size to use. 

Yes,measure the wall thickness of that barrel. I have no idea what the diameter of a 4 bore hole is.
Bob Roller

[bob in the woods]

Bore is .990 inches

[Bob Roller]

According to my little handy dandy calculator that wall thickness is 7/32. I'd go with the 1/4x28 for the additional threads and try for full depth.
Bob Roller

[bob in the woods]

Thank you for your recommendation , Bob.  I believe I have a 1/4 -28 in the shop ...somewhere 

[Bob Roller]

Thank you for your recommendation , Bob.  I believe I have a 1/4 -28 in the shop ...somewhere 

You're welcome.I case anyone is wondering how that figure was obtained,it's simple math. I used the breech
diameter of 1.375 and subtracted the .900 bore and the divided that number by 2 and came up with .2375
so the 1/4x28 if threaded to full depth or close to it should stay in when the gun goes off.Another hint,
counter sink the hole you'tr going to tap to the depth of ONE thread,That will make the tap easier to start and leave
a better looking job as well.
Bob Roller