Tale of two Mausers...Whitworth thread fitment ?

[tobnpr]

Customer dropped off a couple of his Mauser "projects" a few weeks ago and I'm now getting into the "find out what I'm dealing with" stage.

One is a Brazilian receiver, and a Shilen short-shouldered prefit (.300 WM) that's to be fitted with a DBM into a Parker-Hale stock
Other is an Israeli (7.62) FN, that had an original military- but not original to this receiver- barrel partially screwed into the receiver as a replacement for the original.

I found out why the barrel was only partially screwed in- it won't go further than two threads in before it locks up.

There was some light galling in the Israeli receiver from trying to force this barrel in- but the Shilen screws in easily so I know it's not bad enough to cause this fitment problem. The Israeli barrel screws in a bit further into the Brazilian, but still no cigar. This tells me the bulk, if not all, of the issue is the Israeli barrel and not the receiver.

Tenon major is 1.101 on the Israeli barrel (just slightly oversize), and the Shilen is 1.089. Not having a 55 degree thread mike- nor one other than Starrett I use that only goes down to 14 tpi, I figured I'd use the Starrett to get a comparative but inaccurate pitch diameter. Israeli was .011 larger- almost exactly the difference in OD.

So now the question is how to fix this. I have no idea of how this is going to clock, but I gotta get it to fit the receiver before I can do anything further. So...

Would you try skimming the crests first? My thinking is this might solve the issue...
If I do that, and find out the roots in the barrel aren't deep enough- I guess I need to find a Whitworth insert to pick up/chase the threads deeper?

 

[clowdis]

Skimming the crest of the thread won't work. You can set the barrel up in the lathe (indicate off the chamber) and pick up the thread and chase it down until the action screws on. Not the easiest thing to do so make sure you know where you're going before you start. There are enough thread differences in Mausers that I would be afraid to produce threaded barrels for them, but companies still try.

 

[tobnpr]

^^^
Variation is probably why Shilen undersizes the tenon major OD slightly.
I've had the same issue with a couple of McGowen Mosin-Nagant "prefits". Too much variation in receiver dimensions.

With no way to measure the major diameter of the (female) receiver thread, I think I'll skim the crests to turn the major OD down to match the Shilen since I know that OD will work, then pick up and deepen the threads a few thou at a time until the receiver fits correctly. Now to find a 55 degree insert...

I didn't expect this barrel to clock perfectly anyway, so setting it up/indicating was a given to be able to machine the inner and outer shoulders, breech and deepen the chamber to get it to clock and headspace correctly. Just a little more aggravation, it's this love/hate thing with milsurps.

 

[Carl S]

You could turn a piece of scrap barrel stub
To the desired od.one doesn't want the threads bottoming out on each other.a few
Thousands shy hurts nothing. grind a piece of hss to 55 degrees cut thread until action screws on nicely.take note of your machining details then apply them to your
Barrel.

 

[clowdis]

If you can't find a 55* cutter, a 60* one will work fine. If I use a 60* cutter I like to tighten and loosen the action several times before I install it permanently. I don't notice any real difference in the indexing of the action but it just makes me feel better.

 

[tobnpr]

^^^
Thanks- I was wondering about that. I didn't check the Shilen prefit, but my guess is it's 60* like most of the aftermarket barrels. I'm assuming I'd want to align the cutter to remove material from both flanks equally (as equal as I can eyeball it, anyway).

I could grind a 55*, or order one pre-ground from Brownell's, but since 60* will work I'll take the path of least resistance .

 

[tobnpr]

So, who can think outside the box here?

Forgot about the rear sight base (and I'm not pulling that...)- I'd need a 2"+ spindle bore to accommodate the barrel through the headstock. If I remove the front sight, I might barely be able to grab the muzzle end in the outboard spider- but that'll leave me with 5"-6" of stickout on the inboard side which isn't going to work. I've got a blueprinting jig for receivers/short barrels but it doesn't have a bore that large either.

So, I'm "between centers".

All I can come up with is a sacrificial sleeve like I've used for crowning when I've had to work in the steady.
Sized a bit larger than the tenon to slide over it and fit between the barrel torque shoulder and the rear sight base. Center it up by eye best I can, fill it with epoxy. Hold the muzzle end in a set-tru chuck, and the chamber end in the live center. Skim cut the sleeve so it's true to the bore (at least as true as it gets between centers, which is good enough for this rifle), then run the sleeve in the steady to be able to do the machining work. To remove the sleeve, judicious heat to soften/burn out the epoxy so the sleeve itself can be slid back over the thread tenon.

Any other ideas?

 

[Ggmac]

just set up the muzzle end in the jaws and use a steady near the chamber and chamber end in tailstock. If you’re that close I woild use a triangular file and turn barrel , use the file , Ive used small jewelers files , and since the threads are in the tailstock you can check the fitment easily. Sounds unethical but it works and an old smith showed me this .

 

[tobnpr]

^^^
I've tried taking that shortcut before, but without a trued surface running in the steady it's no-go.
No different than a new barrel where the shank gets skim cut to get it true to the bore before it's placed in the rest. On a finished barrel there's no way to do this without using a sleeve so a truing cut can be made beforehand.

If it were just the threads I could leave the chamber in the live center and work it from there, but I know the odds of this thing clocking correctly are about zero which means I'm going to need to face both shoulders and the breech, and then bring the chamber back to depth.

 

[Ggmac]

^^^
I've tried taking that shortcut before, but without a trued surface running in the steady it's no-go.
No different than a new barrel where the shank gets skim cut to get it true to the bore before it's placed in the rest. On a finished barrel there's no way to do this without using a sleeve so a truing cut can be made beforehand.

If it were just the threads I could leave the chamber in the live center and work it from there, but I know the odds of this thing clocking correctly are about zero which means I'm going to need to face both shoulders and the breech, and then bring the chamber back to depth.

Ive done it too many times to count . It was a normal procedure when fitting mauser barrels to customers receivers . Barrels from many different sources were always different. I remember one company that had a unfinished walnut stock ,a Santa barbara adjustable trigger , a barrel in your choice of cal. , and a checkering tool , forgot they also included an aluminum hinged trigger guard all for $99 . I think the companys name was Sherwood . Advertisements in most magazines . They always were snug and needed a little taken off the threads . Just a quick pass with a triangle jewelers file or two and itll screw on . Cant forget the white lead lubricant !

 

[tobnpr]

Well, dayum...
Set it up, and I couldn't believe the indicator- just .002 TIR ahead of the tenon.
I've had brandy-new mid-tier barrels that weren't that close, for an old milsurp I'm amazed. I usually find they're out at least 8-10. Guess FN was on the ball back in the day...