This is a 10x32 thread vent liner installed in a 1" tapered to 7/8" 54 caliber pistol barrel.
Tools needed.
Small center drill.
#53
#44
#38
#37
Drills
Lathe bit for 60 degree V thread.
#2 piloted counter bore, pilot is about .083". Cuts a .155" hole.
10x32 taps, a taper to start and a bottoming tap reground to cut very close to the bottom of the hole.
Drill press and lathe large enough to turn 5/16 stock.
1 piece of 5/16" 303-304 stainless or other material.
Magic marker in blue or black or???
Mark and drill the vent location with the #44 drill through to the bore. Counterbore deep enough so that there is enough metal for a shoulder and at least 4 threads. I leave just a little more than I think I need since I will surely cut into it slightly with the bottoming tap and I then retouch with the counter bore. to recut the flat bottom of the hole. After all counterbore work is done drill the hole to #37, on size larger than the counterbore in the liner.
Counter bored and ready to tap. Measure to the bottom of the counter bore.
After its tapped I *lightly* countersink. Just enough so that the 60 degree on the liner will blot out the threads
Liner is turned to the diameter OD of a 10x32 and a 60 degree angle is applied to seat on the countersink exterior of the counter bored hole in the barrel with the threading tool.
Liner is center drilled and counter bored with a #38 drill. Then drilled another 1/4" with the #53 drill.
Thread with lathe or with a 10x32 die using plenty of cutting oil. Go easy and if you cut into the 60 degree recut.
Cut the threaded portion of the liner to a length that is a few thousandths longer than the hole is deep to the start of the 60 degree. The extra length can be adjusted to reduce the amount of fitting but make sure its longer.
Measure the counter bore in the vent so that you KNOW that the .059 hole will be about .020" long when the liner is stalled. This is guesstimation. And requires some head scratching and more measurement. But the end of the #38 hole needs to be just short of the 60 degree at the point of the drill. Using the drill itself as a guage in the hole can be a help here.
Cut the 5/16 rod about 1"-2" past the 60 degree for a handle.
The fitting is in process magic marker used to show contact against the inner shoulder and the outer counter sink.
This liner can be installed without a counterbore but it will not look quite right.
File/polish the threaded end until the 60 degree sinks into the countersink.
Once the liner is in deep enough in the counter sink and the "blue" is marked inside and out with full contact. You are done fitting.
Cut and file to fit the exterior.
Shadow is right at the end of the internal counterbore.
FFFG powder at the vent.
Why only 10x32?
The breech face is right at the vent. A larger one would be WAY into the breech plug.
Yes I could have placed the lock a little forward but I put it where it looked best to me.
This vent design did well in the speed testing by Steve and Larry and has worked well for me in the past in speed and reliability.
It removes little metal at the breech. The #37 hole reduces the pressure that can be applied to the vent compared to full diameter 10x32 of a #12 or 1/4" vent.
If you check the square inches of a .104 hole you will find its pretty small. If my math is right about .008 square inches. Pressure is surprisingly low on the vent threads.
This is kinda spur of the moment so I may have missed something. I did not plan on a tutorial when I took the photos.
Don't ask how the hole in the barrel got bigger than the pilot on the counter bore before I tapped the hole
Do as I say not as I do
Or YOU TOO can make a sleeve for the pilot on your counterbore.
Dan
PS
This is a #12 hole in a 7/8" x 50 cal smoothrifle barrel. Stood proof OK and 100 rounds or so of testing. It is pretty thin at the bottom of the counter bore though.
