# Measuring internal Threads



## Allan (Apr 28, 2019)

I have an old Pieper Rolling block rifle action that I want to put a barrel on. I have no original barrel. I am thinking of putting an N0. 4 Lee-Enfield barrel on it. Cut it short, turn it down thread it and rechamber it for 32 S&W Long. Bullet diameters are the same. Problem is I am not sure what the OD of the threads should be. How does one go about measuring the threads on an ID of a hole less than 1" diameter? I don't think the three wire system will work.


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## jwmay (Apr 28, 2019)

Here’s a crazy idea. Turn a soft wood plug just a little larger than the hole you want to measure, (maybe even turn a tapered plug) and screw it into your unknown threaded hole. The softwood should yield to the steel threads. When you take it out, you should have a near identical threaded plug for that hole. Now you have something to measure. I know very little about guns by the way, so I’m making an assumption that the plug would be near enough for you to identify the thread, if this type of thing is standardized. Terrible idea?


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## Allan (Apr 28, 2019)

Worth a try. Not sure how accurate it would be. Maybe if I used maple...? I was thinking along similar lines anyway.  Making a steel plug and working it down until it fits the hole nice and tight. Then measure with 3 wires and duplicate the thread on the barrel.


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## RJSakowski (Apr 29, 2019)

MSC and other suppliers sell facsimile compound designed to duplicate hard to measure surfaces but it is expensive. Your idea to make a steel plug to fit is probably your best bet.


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## Allan (Apr 29, 2019)

RJ, you have me thinking. I wonder if JB weld would work if the threads were well waxed? Don't know if it shrinks or not.


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## RJSakowski (Apr 29, 2019)

Allan said:


> RJ, you have me thinking. I wonder if JB weld would work if the threads were well waxed? Don't know if it shrinks or not.


I would be hesitant to try JB Weld in a gun barrel.  You could end up with a mess.  Something that might work is a bismuth alloy.  Cerrosafe is a bismuth alloy that melts in boiling water. https://www.ebay.com/itm/Cerrosafe-160-190-Chamber-Casting-Alloy-1-2-lb-ingot-/301891304226


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## Allan (Apr 29, 2019)

RJ, Cerro Safe is a great plan and I think I ordered som ea few years ago and forgot about it. This reecisever would take a lot of metal since the barrel goes a little over 2" into it. I'll see how much I have. I bought it to check the chamber dimensions on my Ross rifle. Guess I should get on that project, too.


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## MarkM (Apr 29, 2019)

I don t own a rifle so looking at this just as mating threads.
The I. D. Of the threaded hole diameter isn t where you get your fit.  You can stray off somewhat and make no difference.  Obviously you can go too far off and make it impossible to attain a good thread but a few thou under or over what you measure will probably not matter.  Common sense here. Not bigger than your major and a few under will be fine.  You can easily measure the pitch of the thread with normal pitch guages.
Your pitch diameter and where the two mate from the internal and external thread is what matters and gives you your fit.
Measure your i.d. bore out your hole and start threading.  Make your own guage by threading the opposite wether i.d. Or o.d. Then use that thread guage you made for your final part.  Just thread until your own guage has the right fit you want than have at her or buy a set of go and no-go,thread guages for the class of thread you want.


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## Allan (Apr 29, 2019)

Thanks for the reply, MarkM. 

You are right it is just about mating the threads. But I think you may have misread my question. You say, "measure your i.d. bore..." That is what I need to know how to do and what I was asking- how does a guy measure the major diameter of a hole less than 1" in diameter let alone determine the thread pitch and form? 

Since this was a Belgian made turn of the previous century firearm the thread is unknown. It seems to use various threads. Again, since the bore is so small it  is hard to get a read on the thread pitch and profile. One cannot see the thread pitch gauge perpendicular to the gauge. I have played around with it a bit and think I have the thread figured out but still have some niggling doubts. 

The idea of a cerro safe casting would make that thread determination a lot easier. It would also make the major diameter of the threaded hole easier to measure.

In any case I would make up a plug and turn the threads a bit at a time, as you suggested, until the final fit is attained. Then I can measure with 3 wires and thread the barrel by sneaking up on it.

Gun threads are a weird mix. German built Mausewr action use a Whitworth thread. Many smithys just use a 60 degree thread and force them to fit. Seems a bit bush league to me. Many have weird thread pitches like 6-48 and some can be metric or just about any thread that was ever made. One thing that is certain is that you can't assume.


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## MarkM (Apr 29, 2019)

Re read again after erasing my first sentance.  Sorry!  Off on a Tangent I guess. 
You know being out of Imperial or metric was the furthest from my thoughts.  Took for granted being metric.  
Could you not just use a caliper.  I know it s not as precise but one good with instruments can do a decent job for the tolerance needed for threading.  Just my thoughts.
How about using some type of putty or even playdough to figure your pitch and the angle of the thread.  Then with your I.D. Measurement you could go to Machinery Handbook and get your proper numbers armed with enough information.  Just throwing it out there!


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## Allan (Apr 29, 2019)

I have no hole to bore I need to thread the outside of the barrel to fit the threaded hole. I don't know how I can get a caliper to read the valleys of the hole thread. I think for the sake of accuracy I'll go with cerrosafe. I already am pretty good at building in variables by accident so I don't want to engineer them in. LOL.


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## MarkM (Apr 29, 2019)

Not a Vernier caliper. Old school transfer caliper.


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## Latinrascalrg1 (Apr 29, 2019)

I would think you could make a very good copy using hot glue!  Apply sometype of release compound first then you could probably place the end of a gluestick into the barrelhole and gently heat the Reciever until the glue softens and flows into the threads. Put in the fridge for a few minutes and it should unscrew right out.  I would probably find a way of blocking off the back side so that the glue stays in the threaded area but if that's not possible you should be able to make it work either way.


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## rgray (Apr 29, 2019)

12L14 is pretty inexpensive. 
I'd thread as many chunks of that to what I thought it was till I ended up with a good fit.
Then thread the barrel to match your new "thread gauge".
In the few mausers I've done I've run into 4 different thread sizes. A couple of them I think were intended to be the same thread but what the operator put out that day was not on target.


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## MarkM (Apr 29, 2019)

Don t use the caliper to measure with but go in with a pre determined size.  Set the pc. So its vertical with light coming through the bottom.  Set the calipers and rock on the thread with your pre measured and set caliper.  Use it like. Go no go guage then go verify with some aluminum.   
Lightly oil the thread and lay putty on it.  Pull it out and get your pitch and angle of thread.  Have at her!


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## Allan (Apr 29, 2019)

Thanks for the suggestions. Time for some mulling....


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## mikey (Apr 29, 2019)

I would use an inside micrometer to find the ID. The jaws span the thread peaks and give a better reading. The problem is that you probably don't own one of these contraptions so the next best option, for me anyway, would be to take a piece of round stock and turn it down until it juuust slips into the bore and measure that. We know that production tolerances for threads can be rather coarse so we cannot rely on a standard; using a plug gauge you make would at least get you within a few tenths if you work carefully. Then you'll at least have an idea of what the minor diameter is. Then you can sort out the pitch with a pitch gauge and you'll have your thread.

It's going to be fun sorting out the thread form. I'm not a gun guy but figuring out if they used a 60 degree thread or a 55 degree Whitworth could be challenging to do by eye. Here I think the Cerrosafe stuff might come in handy.


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## jwmay (Apr 29, 2019)

I remember reading an article on that stuff(cerrosafe) or some equivalent. The guy was doing something with the receiver of a rifle. Wish I remembered what. Anyways, I think that’s a really good idea. I had the distinct impression from the article that the author was happy with it.


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## Allan (Apr 29, 2019)

It looks to be a 60 degree thread. And I now forget what pitch I thunk it was. I'll post a picture or two just for funzies.


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## Allan (Apr 29, 2019)

So here is the action and barrel. I need to turn the OD of the barrel down to the major diameter of the barrel hole in the receiver. The two cartridges show the approximate location of where they would sit in the chamber. So I will have to cut off the breech end at about where the breech end begins to taper into the barrel proper. If you look carefully you will see a hole drilled through the barrel in order to DEWAT it.

For now I need to determine the major diameter of the receiver threads to see if there will be enough meat on the barrel fully fit the full length of the receiver. I will need to have a face that rests against the front of the receiver to act as a depth and headspace stop.

The bullets are the same diameter as are the neck diameter on the 303 and the case diameter on the 32 S&W Long (or 32 Colt New Police, if you prefer). I hope I can get it to work.


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## Allan (Apr 29, 2019)

Not sure if the 1 in 10 twist of the Enfield barrel will work. If anything it will be a tad on the fast side so I anticipate it should work OK on a slow mover like this. Kind of like windmilling a soft ball, only slower and not as flat a trajectory.


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## Bamban (Apr 30, 2019)

Here is TSJC compilation of barrel shanks. Yours might be in this document.


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## Allan (Apr 30, 2019)

Bamban, that is a cool resource. Lots of stuff. I see that the angle of the V is not usually included. I have similar chart out of American Rifleman in the 60's but it is not as complete as this one. Thanks for sharing. Unfortunately the Pieper is not in it. It was not a very common or particularly high end rifle. Probably not worth much effort but I would like to see it live to shoot another day. I'm a sucker for old stuff and keeping them going.


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## derf (Apr 30, 2019)

I'd use cerrosafe, but you don't have to pour the whole length. Plug up the bottom with play dough so you only have about 1/2" to pour. Coat the threads with Vaseline. While you are pouring, stick a screw driver in the middle so you have a way to unscrew it.


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## MarkM (Apr 30, 2019)

Earlier on when I said I was just throwing it out there.  Sometimes I think just a different train of thought or like walking away for five and such so here is another I ll throw out there.
Could you not bore the hole and thread to what you can see from the outside?  Or is there just not enough meat there and you d compromise the action?
Can I ask not knowing much about Gunsmithing?  Is the reason there is use of such fine threads due to the fact that sometimes close enough has to work like someone stated ealier where threads were crushed together with the use of stuff maybe from another century that hobbyist would be interested to use to modify?  Or am I out in left field?


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## aliva (Apr 30, 2019)

Check Tom Lipton's video. He uses a material called woods metal to make a positive casting of a steering wheel spline. This might works for you. You should be able to unscrew the casting once its cooled and then take your measurements.
About 2minutes in to the video 



Wood metal https://www.belmontmetals.com/product/woods-metal/


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## stupoty (Apr 30, 2019)

Allan said:


> Since this was a Belgian made turn of the previous century firearm the thread is unknown. It seems to use various threads. Again, since the bore is so small it is hard to get a read on the thread pitch and profile. One cannot see the thread pitch gauge perpendicular to the gauge. I have played around with it a bit and think I have the thread figured out but still have some niggling doubts.




If their an unknown thread on an antique I would do some research, https://en.wikipedia.org/wiki/Belgian_units_of_measurement  , their it says they were metric since 1820 ish. 

So it's probably metric. if you can get a thread pitch gauge in their that should be ok to figure out by feal.  You could then try making test threads and screwing them in.

Stu


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## Allan (Apr 30, 2019)

Hey. Mark.
Some threads on barrels are quite fine V threads and some are quite coarse square threads. and everything in between. Check out the list that Bamban posted. I think the 1905 Ross has a double start 8 per inch square thread. What their reasons were is anybody's guess.

Stu, it seems that what was official measurements went out the window when it came to guns. As noted previously the German Mauser 98s used a British Whitworth thread. I think I'll try Cerro Safe and see for sure. A small plug of it as Derf and Aliva said with a screwdriver or something placed in it for removal seems like a pious idea. I'll check the video as well.

Thanks you all for your suggestions on such an arcane subject.


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## Allan (May 3, 2019)

Well I poured some Cerro Safe but it don't turn our very good. I guess the receiver was not warm enough in spite of heating it before the pour. I'll try it again tomorrow with a heat gun trained on it. 

It looks like the thread is 18TPI Whitworth but its hard to tell the difference between 55 and 60 degrees on such small threads tucked inside a small hole. Maybe tomorrow's pour will work.


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## Allan (May 3, 2019)

OK. Got a plug made and measured. Having it out helps immensely. I have determined that it must be a British Standard Parallel Pipe Thread
 19 TPI. How weird is that?


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## westerner (May 3, 2019)

Allan said:


> How weird is that?


Very, in the world I run around in


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## Allan (May 3, 2019)

Good thing I ran the casting  or I would have assumed it was 18 TPI and made up a test plug only to have it jam. That would be my usual approach. Here is the Cerro Safe Casting I made.


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## BGHansen (May 4, 2019)

Allan said:


> I have an old Pieper Rolling block rifle action that I want to put a barrel on. I have no original barrel. I am thinking of putting an N0. 4 Lee-Enfield barrel on it. Cut it short, turn it down thread it and rechamber it for 32 S&W Long. Bullet diameters are the same. Problem is I am not sure what the OD of the threads should be. How does one go about measuring the threads on an ID of a hole less than 1" diameter? I don't think the three wire system will work.


Here's something I posted using epoxy putty to make an internal thread plug.  Did basically what you suggested, lubed the inside with LPS 1 lubricant, mixed up the epoxy putty, mashed it in and unscrewed it.  Easy peasy, no issues at all.

Bruce

https://www.hobby-machinist.com/thr...did-you-make-in-your-shop-today.67833/page-51


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## Allan (May 4, 2019)

Thanks, BG.
How dimensionally stable is the epoxy you used? Does it shrink or expand much?


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## Allan (May 4, 2019)

Was going to say that one thing I like about Cerro Safe is that if it sticks you can just heat it up and melt it out and try again. I also did a chamber cast on an old Ross rifle yesterday. Never done one before and the thing would not come out although the instructions say to push it out with a cleaning rod. Much heavy pounding would not dislodge it so I melted it out. Apparently it has throat erosion from the old cordite powder and the erosion ring was deep enough to lock it in place. Had it been epoxy it would have been real challenge to get it out again given how inaccessible it was.


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## BGHansen (May 4, 2019)

Allan said:


> Thanks, BG.
> How dimensionally stable is the epoxy you used? Does it shrink or expand much?


According to the data sheet shrinkage is <1%. That'd be less than 0.007" in my case.

Bruce


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## Allan (May 4, 2019)

Not bad but a bit much for my critical dimensions. But maybe not. I still have to start a test plug on the large side and work down to a fit anyway.
Cerro Safe shrinks slightly at first but within an hour expands. It will eventually stabilize at + .0025" on something like cartridge case.


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## Allan (May 5, 2019)

Good thing I ran the casting I would have assumed it was 18 TPI and made up a test plug only to have it jam. That would be my usual approach.


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## derf (May 6, 2019)

So how would you cut a thread of 19 tpi?


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## Allan (May 6, 2019)

Actually my lathe has a setting for 19 TPI. I was a little dubious so I checked it out. I guess that 19 TPI 55 degree thread is still used all over the world so these older lather must have been set up with that in mind. Kool.


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## derf (May 7, 2019)

Well, that is surprising and cool. I guess you just need to thread up scrap piece and test fit it.


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## Allan (May 7, 2019)

My thoughts exactly, Der. But spring has sprung and other projects will put it on the back burner for a while. Maybe a rainy day will intrude.....


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