How to determine unknown internal thread?

[JRD]

I've got a project gun that is missing some screws. It's an early 1900's German made double barrel shotgun. The floor plate screws from the action are missing. It was someones project and the screws were lost before I acquired the gun.

Quality of the gun makes it worth my time to put the TLC and effort into repairing it.

It's easy enough to figure out a external thread when you have the screw, but when you only have a tapped hole, how do you determine the thread?

Being pre-WWI German, I'm betting it's metric, but is also potentially a custom thread from before the days of standardized fasteners.

I can get an idea on minor diameter by using drill shanks, would assume it's a 60 degree thread form, but how can you tell the pitch?

[Cap'n Morgan]

Make a dummy screw of soft material - plastic, hardwood, aluminum, whatever. Make the "screw" slightly larger than the internal diameter of the hole, just enough that it will take on the threads. Screw it in a few turns and measure the length/height of the screw outside the hole with a caliper. Now turn the screw an additional number of turns (count them!) into the part and take another measurement. Subtract the last measurement from the first and divide the result with the number of turns - this will give you the pitch.

[LAGS]

If the screw is of reasonable size, then take a piece of rod that will slip loosely into the hole.
Coat the internal threads with wax or Johnsons Paste wax, ( A Thin Coat )
Now coat the tip of the cleaned rod with Epoxy, J B Weld or bedding compound.
Then insert it into the hole, and let it fully cure.
You can then unscrew it and the epoxy will give you a rod with external threads that you can view and even measure.

[JRD]

Thanks for the suggestions. Those are two very logical ideas. I'll probably start by turning down a piece of delrin and trying to thread it in.
I'm a little leary of JB weld. With my luck I'd forget the wax and epoxy the rod into the action.

[LAGS]

Warm up the Plastic rod, and see if the action will act like a Die and actually form the threads on the rod.

If you ever epoxy something like that, you can warm the receiver up with a heat gun or hair dryer to break the bond.
Or if the part is small enough, put it in the freezer to break the bond.

Dont ask how I know these things

[W.R.Buchanan]

This can be a problem however if you just start trying different size screws in the hole you may find something common that fits the hole.

Unless you have an Optical Comparator to very accurately measure the thread pitch you won't be able to measure it close enough to discern between Inch or Metric threads that are only a few thousandths apart as many small metric threads are very close to SAE threads.

Example: 10-32 is generally called out on Military Drawings as .190-32. 5MM-.8 is actually .196-32.25

The Germans didn't really fully commit to Metric threads until WWI. Prior to that they used whatever was close.. Barrel threads on Mausers are not metric, they are bastard inch threads. like .920-16 or something like that. I don't have my Mauser book in front of me right now or I'd get the exact numbers.

The Ejector Screw on an #4 Enfield is 8-37 as opposed to an 8-36 NF thread, but an 8-36 screw will go into the hole just fine because as long as it doesn't have to go deep enough that the extra thread causes it to bind. Also if you run an 8-36 tap thru the hole it takes care of the problem for good. 8-36 Screws are not easy to find and most outfits don't even list them any more as they are so close to an 8-32. However 8-37 screws are not available anywhere!

If I was you I would try running some taps thru the hole until you find one that is close and common. Then I would convert it to the common thread that you can readily buy screws for.

It's not like it would destroy the value of the gun because you are the only person who would know that it wasn't stock. I seriously doubt anyone could ever tell the difference as long as the head of the new screw looked the same as the original, nor would they even have a reason to check it.

Randy

[Ballistics in Scotland]

Most screws conform pretty well to the rule that the major diameter is about equal to the minor diameter plus the pitch, 12mm. x 1.0 having a clear inside diameter close to 11mm. It only goes far wrong if the threads are more or less truncated at the crests and roots than usual. You can very often find the pitch from the dark marks left by oil and dirt on a pale coloured wooden or plastic rod.

If a short thread is of the right diameter and just slightly incorrect in pitch, you can very often get by by screwing it in as far as it will go, tapping quite hard on the end, and giving it an extra bit of turn. I have had this work well for nipples in my Bohemian double percussion rifle, which actually were ¼in. in diameter, but with a metric pitch, although I can't think why.

A shotgun action may well be case-hardened, which is difficult to tap well, and some threads are both special and finer than you will find in a tap. A lot of unusual fine taps, both Imperial and metric, can be bought on eBay from China, and while I don't know if they are of really high quality, I have found them fine for occasional use.

Gun screws often have finer slots than those you can buy nowadays, and there has been a tendency to go over to torx scope fitting 6-48s etc. To cut fine slots I like a hacksaw blade from which I have ground the left and right waviness of the teeth which normally keeps the saw from binding. This is also good for cutting V-springs with the minimum of weakening roughness inside the V.

[country gent]

Epoxy can be used besure to get a good coat of release agent in it. Cero safe can also be used to make a cast of the threads. Build up around hole with clay to help it into the hole and give something to turn it out. We did it on old equipment at work with lead even

[JRD]

Thanks for the additional suggestions and information on the history of threads gentlemen.
I have enough to go on now!

[country gent]

American standard threads are 60* included angle on the form while a lot of European threads are 55* threads.

[KCSO]

Pour in some cero safe and viola you have EXTERNAL unknown threads.

[Andy]

I made a bolt i.d. kit by buying 1 ea. of every semi-common nut and bolt size/pitch there is, up to 3/4", and labeling them in compartments in a large fishing tackle box. It costs about $75 to get all the common and most uncommon SAE and metric sizes off boltdepot and they will ship them in individually marked bags for you.

I realize this doesn't help if the thread is a non-standard custom thing, but it's a handy thing to have around for these sorts of situations

[leebuilder]

I braid soft solder and screw it in the hole. I can get a very accurate pitch measurment most times a good idea of the major diameter.
Be safe

[fast ronnie]

You might want to check to see if it is Whitworth. Mauser bought English machines to build rifles. I have a well known gunsmith who swears they are metric, but are actually whitworth. I work on Hawker Sea Fury parts which are 55 degree thread angle, not 60 degree. They also have a different designation for the diameter of the threads. Their thread sizes do not relate to the diameter of the threads. In the United States there were many different oddball pitches used, also. I also work on a number of U.S. made motorcycles before 1929, and there were no standardized thread pitches at that time in the States. Sometimes it can be a real problem. (Mauser large ring are 1.100 dia.)

[comdiver]

Many years ago there was a guy that made canons as a hobby. I've been told he was one of the best in the country. Sometimes I could get Don to look at one of my projects. Anyway one of them involved measuring a internal thread. He very simply walked over to his wood pile and grabbed a small piece of wood which he put it in the hole and pressed it into the threads. When he removed it he could compare it to a thread gauge. Simple. You could also try different screws. Just don"t force them.

[funnyjim014]

What about light oil in the hole and hot glue. Let it cool and thread out