Measuring internal Threads

Allan

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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 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?
 
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.
 
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.
 
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.
 
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.
 
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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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.
 
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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