Question about 1895 Marlin barrel thread.

[modified5]

Ok, here's the deal. I picked up a sweet 1895 1\2 octagon 1\2 round barrel in 45-70 for my Darcy falling block project.
It has square threads of course. I measured the thread major and it comes out to .780ish.
Converting that I get 25\32 or 19.84mm.
My question is for those who have cut threads in a receiver for these barrels. What tap did you use? Where did they come from?
I'm having trouble locating something.
I have considered turning the treads off on my lathe and cutting my own, but losing that much of a safety factor makes me a little nervous. I'm kind of attached to my face!
Anyway, if someone can help point me in the right direction I would appreciate it.
Thanks, Scott.

[ulav8r]

Haven't used one of those, don't know the spec. Instead of tapping the receiver, it should be internal threaded on the lathe, with the thread fitted to the barrel.

[JimB..]

I've never seen a tap for square cut threads, but then I've never looked.

You might find some of the comments in this old thread useful. http://castboolits.gunloads.com/arch.../t-108838.html

[Moleman-]

If you have a lathe that will cut threads that is the way to go. I believe the TPI is 12 and I'd just grind a bit that blocked out most of the light when placed between two threads on the barrel. Many ways to do it, but I just set up a dial indicator to tell me how deep the bit is going into the hole. Set the compound to zero degrees and only feed in towards you with the cross feed. Then cut the thread relief (thread stop or whatever you want to call it) to full depth. Cut the threads and test fit the barrel. If the threads don't fit you have to determine if the major dia isn't wide enough or the thread isn't wide enough. If I got a few thousands over major dia and the barrel still didn't fit you could advance the compound .002" and recut the threads like you did before. I've only cut a few internal square threads and had no issues, but it's always good to think about what you're going to do should you hit a snag. Here's a threading bushing for my late 1880's to early 1891 E.E. Garvin Mill I made. It's a 7/8"-3TPI-2 start thread. Had to replace a 22t input gear on my lathes gear box with a 11t gear in order to cut 3tpi as 6tpi was the lowest setting. Otherwise you're looking to find a 25/32"-12TPI square thread (not acme) tap that may or may not work if the threads are a modified square.

[jmorris]

Found mention of the threads here but didn't see a tap available in a quick search of usual places.

http://castboolits.gunloads.com/show...st-info-needed

You could always cut and rethread the barrel to something you can find a tap for and re chamber when your done. Would be cheaper to thread on a lathe after grinding some HSS to the proper shape to cut the threads.

[modified5]

I am going to look at a 20mm 2.0 tap or die. If the threads are shallow enough they may cut down the square threads to single point.
Or screw the whole thing up.

[JimB..]

I hope that you got the barrel cheap! So why don't you contract out threading the receiver, there is no shame in it, and far less risk.

[modified5]

Jimb, you are probably right. I am still in the gathering info\material stages for this falling block build.
I am going to build a 30-30 version first. I have a 94 Winchester barrel to use.
The nice thing about them is they are a common 13\16 20tpi thread. Taps are readily available.
When I get ready to build the 45-70 I will get really serious about dealing with this thread issue.

[MostlyLeverGuns]

Barrel threads are normally cut on a lathe. Brownell's has a few taps that have barrel threads, LR Mauser, SR Mauser, others, most are not meant for installing barrels - more for correcting damaged threads. Square threads are cut using a cutting tool ground for the correct pitch, most are 12 tpi or 10 tpi, on a lathe. Major and minor diameters are carefully measured. To cut threads in a receiver, a thread cutting tool on a lathe boring bar is used in a 'non-production' line or custom environment. You could copy the threads in good steel using a lathe, then create your own tap. I believe Dave Manson Reamers makes a set-up for the Marlin's to enlarge and re-cut Marlin threads for cases based on the .348/.50 Alaskan.

[W.R.Buchanan]

It is an ACME thread and if you goto the Machinerys Handbook you can probably find it. Then once I identified it, I would go to MSC and look thru their catalog as they have just about every single tap known to man.

ACME thread taps look like really long taper taps, and progressively open the threads up until full diameter is achieved. Since the Thread Profile is square or trapezoidal and most of the cutting is done at the ends of the threads, the depth of the thread must be cut in small increments,,, hence the long tap.

Whereas it is possible to turn those ID threads on a lathe it would be a very challenging project and I would recommend having some spare underwear handy. I have cut Acme Threads on a lathe but they were a lot bigger than these and they were also in bronze.

A better way to go would be tap the receiver to a known size and cut off the existing threads on the barrel and rethread it to match the receiver. you'd have to rechamber it but you would retain your safety factor.

Randy