Odd powder drum threads

[SandWatcher]

I'm replacing a corroded powder drum on a .45 rifle barrel, manufactured for a kit in the 1960s or 70s. The only markings are "Spain" and a six digit serial number. I have a some new drums from ToW of various sizes (including a CVA drum), but nothing matches what came out of this barrel. I've measured the original several times, and it looks like it needs 3/8-20 threads. Has anyone heard of such a thing? Am I going to need to fabricate a replacement from scratch?

 

[Colonial Boy]

As it’s a Spanish gun the threads will, almost certainly be Metric.
3/8 , 20 is a BSF size

 

[smokeum54]

Been awhile but the last spain/cva i had out was 10x1.25 if i remember correctly.

 

[SandWatcher]

As it’s a Spanish gun the threads will, almost certainly be Metric.
3/8 , 20 is a BSF size

10-1.25 metric seems too large. See comparison to the original in the center.

 

[Dusty_Traveler]

20 pitch SAE threads come out to 1.27mm in metric.

I would run a M10x1.25 tap through it, install the corresponding new drum and rock on.

 

[Sam squanch]

I learned to expect the unexpected on many guns!

 

[ZUG]

View attachment 351934

10-1.25 metric seems too large. See comparison to the original in the center.

From looking at your picture the thread pitch of the original drum looks close to the pitch of the M10x1.25. If the gun was mine, I'd tap the barrel for the M10x1.25. Before I would do that I would measure the pitch with a pitch gauge to actually find out what the original is. Metric pitch gauges are cheap. Amazon has pitch gauges for under $10 and M10x1.25 HSS taps for under $10 also.

 

[44Bro]

It's 9X1.25mm. If the barrel threads are good, call Track or Dixie. If the threads are buggered, do what everybody is saying, run a 10mm tap in and call it good.https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm

 

[longcruise]

I'm replacing a corroded powder drum on a .45 rifle barrel, manufactured for a kit in the 1960s or 70s. The only markings are "Spain" and a six digit serial number. I have a some new drums from ToW of various sizes (including a CVA drum), but nothing matches what came out of this barrel. I've measured the original several times, and it looks like it needs 3/8-20 threads. Has anyone heard of such a thing? Am I going to need to fabricate a replacement from scratch?

At this point it could be anything. It could have been re tapped since manufacture.

Maybe just try threading In various bolts till you find a fit. My hardware store doesn't bat an eye at me taking a gun barrel into the store but check first with them. Then you will have all the possible sizes at your disposal.

Also consider that you may be able to drill and tap for an existing drum for a fit.

 

[SandWatcher]

It's 9X1.25mm. If the barrel threads are good, call Track or Dixie. If the threads are buggered, do what everybody is saying, run a 10mm tap in and call it good.https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm

I think you might be right about it being 9mm. The threads on the barrel aren't pristine, so tapping it up to a 10mm might improve things. I have a thread gauge, but it isn't precise enough to differentiate between 1.25 and 1.27mm. I'm only going to get 3 or 4 good threads in the barrel wall, I'm hoping any difference will be negligible. Here's a close up of the original drum on the left, set next to the new 10-1.25 drum on the right. I think for now my plan is to follow the advice here and run a 10-1.25 tap into the barrel so I can just use an off-the-shelf drum going forward.

 

[Colonial Boy]

If you do get a very close match try running a bolt in before you re-tap as the bolt will tend to move metal whereas the tap will cut thus removing some of the thread.

 

[44Bro]

Three full threads will give full shear strength of the weaker material.