Metric Thread Forms in U.S. Martial Arms

[Rokon]

U.S. martial arms were made with metric thread forms up until the adoption of the Krag. The change came when the Norwegians provided the manufacturing plans for the Krag in the inch-system.

 

[Zonie]

For what it's worth, the Military rifle manufactured by Springfield Arsenal in 1875 that I own uses inch series threads similar to those in common use today.

One of the two lock screws that pass thru the stock to hold the lock in place has threads that measure .187 in diameter. The other one measures .191 in diameter. That is basically 3/16" in size.

Both of these screws have threads that have 27 "threads per inch" spacing. .190 diameter is the nominal size for a #10 screw in the U.S.
The standard #10 screws in use today have either a 24 thread per inch or a 32 thread per inch pitch which makes the Springfield screw's 27 thread per inch pitch a non-standard size.

If these lock screw thread measurements were converted to metric the .187 diameter becomes 4.75 mm and the .191 diameter becomes 4.85 mm. The 27 threads per inch becomes, 0.94 mm. Averaging the diameters and using the .94mm pitch, that would make the metric threads a M4.8-.94 size. The closest metric thread I can find is a M5-0.80 thread.

The tang screw that passes thru the tang and screws into the trigger plate measures .216 in diameter. The thread pitch is 24 threads per inch.
This is a standard #12-24UNC American size screw thread.
A metric equivalent to this thread would be a M5.48-1.06 thread.

During much of the latter 19th century there were no standards for threads but the inch system of measurement in the US had been thoroughly established.
Based on this and the thread measurements I found on my Springfield, I question the idea that metric threads were used on US Military guns prior to the turn of the 20th century.

 

[hawkeye2]

Zonie you are right, the threads are inch though obsolete today. At one time S&S Firearms had the tap for the lock screws though I don't see it now and I have also seen a die offered. Same diameter & pitch screws used on '63 clamping bands. The same screws were used on through the trapdoors and many of the lock parts interchange with earlier guns. The 27 TPI can be found on Colt C&B revolvers and I have an original underhammer with that pitch on at least 2 different screw diameters. FWIW I had to rework an action screw on my 1st. year Winchester high wall and looked for a die to chase the threads. They were an obsolete thread finer than what is in use today but I have forgotten what they were.

 

[Rokon]

For what it's worth, the Military rifle manufactured by Springfield Arsenal in 1875 that I own uses inch series threads similar to those in common use today.

One of the two lock screws that pass thru the stock to hold the lock in place has threads that measure .187 in diameter. The other one measures .191 in diameter. That is basically 3/16" in size.

Both of these screws have threads that have 27 "threads per inch" spacing. .190 diameter is the nominal size for a #10 screw in the U.S.
The standard #10 screws in use today have either a 24 thread per inch or a 32 thread per inch pitch which makes the Springfield screw's 27 thread per inch pitch a non-standard size.

If these lock screw thread measurements were converted to metric the .187 diameter becomes 4.75 mm and the .191 diameter becomes 4.85 mm. The 27 threads per inch becomes, 0.94 mm. Averaging the diameters and using the .94mm pitch, that would make the metric threads a M4.8-.94 size. The closest metric thread I can find is a M5-0.80 thread.

The tang screw that passes thru the tang and screws into the trigger plate measures .216 in diameter. The thread pitch is 24 threads per inch.
This is a standard #12-24UNC American size screw thread.
A metric equivalent to this thread would be a M5.48-1.06 thread.

During much of the latter 19th century there were no standards for threads but the inch system of measurement in the US had been thoroughly established.
Based on this and the thread measurements I found on my Springfield, I question the idea that metric threads were used on US Military guns prior to the turn of the 20th century.

Interesting observation but a faulty conclusion. The original martial "patterns" used by the U.S. were the 1766-1770-1771 French muskets using metric thread forms and the metric form continued to be used in U.S. martial arms production. The question becomes "when" did the metric system fade out in U.S. martial arms production? I was lead to believe that the Krag was the 1st fully inch-pattern U.S. martial arm to be adopted. And then, the metric system returned once again with the adoption and production of the M14 and M16.

 

[hawkeye2]

You'll find a fairly extensive discussion of metric threads vs US threads here:

https://www.muzzleloadingforum.com/threads/musket-dilema.94112/

 

[hawkeye2]

"Interesting observation but a faulty conclusion."

Rokon, if possible could you direct us to documentation that the US arms had metric threads? That information would have to apply to all US arms at least from the early 1800s through the end of trapdoor production and also apply to all the contract arms provided for the US as the screws (and parts) interchange with Springfield and Harpers Ferry production. I haven't been able to find that information to date but if it exists it would help resolve the issue.

 

[Rokon]

"Interesting observation but a faulty conclusion."

Rokon, if possible could you direct us to documentation that the US arms had metric threads? That information would have to apply to all US arms at least from the early 1800s through the end of trapdoor production and also apply to all the contract arms provided for the US as the screws (and parts) interchange with Springfield and Harpers Ferry production. I haven't been able to find that information to date but if it exists it would help resolve the issue.

Hawkeye--Probably the best source in a condensed form are the posts at practicalmachinist.com--a site which really digs down into historical thread forms.
I'll try to attach a file from "Doc AV", which gets right to the point and also covers Zonie's issue about metric/inch combos. It wasn't until 1864, when the American, William Sellers, formulated and proposed the standard U.S. 60-degree thread form with various thread pitches for different diameters which soon became the American Standard Coarse Series (NC) and the Fine Series (NF). Note that date, 1864.

I'm having trouble tonight attaching the file. Give me your e-mail address and I'll e-mail it to you and maybe you can post it to the forum

 

[hawkeye2]

There are also extra fine US threads (UNEF) but I don't know when they came into existence. If the US arms threads are inch they do predate any standardization (they don't conform to the NC & NF series or the UNEF). In the case of the 49 Colt that I examined the threads were very close to a metric pitch but the diameters were way off and on an underhammer I own there are 2 different diameter 27 TPI items, 2 screws and the nipple itself.

 

[Rokon]

Get me that e-mail and maybe we can get the white paper on US metric arms posted.

In the meantime, we have found that the .250x28 nipples fit (adapt to) many antique European arms. The U. S. adopted the 60-degree thread of the metric system and as you begin to run into thread questions, it's remarkable how close some of the metric diameters and pitches are to the later U. S. NC/NF series, virtually offering, for all practical purposes, interchangeability.

 

[hawkeye2]

"for all practical purposes, interchangeability" Sorry but as a former mechanic and auto mechanics teacher of 17 years I have never found a metric or US fastener that would interchange and though some are close the combination will result in damage to one or both fasteners male or female. Yes you can get 2 or sometimes 3 threads to engage but after that you need a bigger wrench. I also spent enough time working on British motorcycles so that I'm familiar with Whithworth, BSF and CEI threads (and tools).

BTW I have sent my e-mail address as a PM

 

[Rokon]

Hawkeye--I e-mailed you the article I referred to from the Practical Machinist on U.S. metric thread forms in the production of our early U.S. martial arms. Hopefully, you can post it here for all to read. It's about as succinct a description as I've come across.

 

[Rokon]

Hawkeye--I'm going to make one last try of posting the Practical Machinist article. If I don't succeed, I'll depend on your superior computer skills to post it to this forum.

Nope, didn't work. Weird, I can attach the file to an e-mail but cannot even bring the file up on the desktop when I try to attach it to this post.

It's in your hands.

 

[Ontario47]

Starting with the goings on in 1776 and only ending as the dust and tempers settled after the war of 1812, the "States" were generally blocked from access to guns, gun designs, and more importantly tooling from England. The only other major source of these items was France. The complete conversion of France to the "Systeme Metrique" started with the Storming of the Bastille in 1789, the same year as the adoption of the US constitution and start of the US martial arms and in general the US industrial revolution. Simeon North (the first Martial pistol maker) used patterns and tooling (in metric) from France. Metric based firearms were common till well after the War between the States.

An interesting bit of trivia: The US, and failing states of Libya and Myanmar are the only ones still officially using the SAE (English) system.

 

[cositrike]

U.S. martial arms were made with metric thread forms up until the adoption of the Krag. The change came when the Norwegians provided the manufacturing plans for the Krag in the inch-system.

Does that mean that the nipples for my 1841 Mississippi and my 1842 martial pistol Are not 5/16 x24?

 

[Rokon]

If it fits, use it. As you can tell, this whole issue is murky with even a mix of metric and fractional thread forms being used. I believe the metric equivalent for 5/16 x 20 nipples used on some reproductions is 8 x 1.25. Is there a metric equivalent for 5/16 x 24? Don't know.

Anyway, the more we dig--the more we learn. That's what's fascinating about our hobby.

Hopefully, Hawkeye will be able to post the article on martial metrics from the Practical Machinist publication.

 

[Rokon]

I'm going to give this one last shot--to attach the article on martial metrics from the Practical Machinist. Nope!

 

[Zonie]

A 5/16-24UNF thread (.312-24UNF) would be a 7.94-1.06 metric thread. A M8-1 metric thread would be a .315-25.4 thread in the inch type thread measurement system.
Like hawkeye2 said, there are some metric threads that are close but after several threads are engaged it takes a larger wrench to tighten them.

 

[Rokon]

Zonie--you'd be surprised how well a .260-28 nipple works in a M8-1 nipple seat

 

[Rokon]

Hawkeye--Are you able to post my e-mail to you with the Practical Machinist article on martial metrics?

 

[hawkeye2]

Finally got caught up and found time to try posting.

Rokon sent me this document and asked me to post it for him.

Standardised "military " Thread systems:
Springfield armoury ( est. 1795) used the French metric system for the manufacture of the M1795 Springfield Flintlock Musket, as it was a derivation of the M1766- Charleville Musket of the Revolutionary War, as Modified by the French Committee of Public Safety, St. Denis Armory, in 1791-92, to use a metric system of threads, based on the earlier french system of "numbered Gun Threads" used by the Factories of Charleville, St. Etienne, etc.

One of the first actions of the Public Safety was to enforce the use of the metric system , and the unification of Gunmaking throughout France, to simplify Gun manufacture for the new Republic. BY 1810, they had a centrally controlled system of QA throughout all the Imperial manufactories, with reasonable interchangeability of most parts from all sources, and a "guide book" for Inspectors on how to carry out these QA inspections of Contractor's work ( M.Dale, M.re Imp. de Liege, 1810, republished by Liege Metropolitan Arms Museum, 1980s).

The Springfield Armory thread system was rendered into "inches" ( Thousandths of an inch)...and this was used by Prof. Sellars of Philadelphia to construct his Unified System Threads, presented to a conference of Engineers during the American Civil War..( all inch dimensions and pitches, but with the metric 60 degree thread form).

The "Springfield metric" system was used up until the adoption the the Krag rifle in 1892, which was designed and made according to the Sellars Thread scheme. ( Thread diameters, Pitches, sizes.)...so one will find some of the M1873/884 45/70 guns will use some truly metric screws, in exchange for the Springfield ones.

Given the sequential commonality of all the US Thread systems in Gunmaking for Military use, it is no great surprise that some Modern screws will interchange with the more "ancient" ones.

Regards,
Doc AV
AV Ballistics Film Ordnance Services,
Brisbane
Australia.