Guard Screw Threads and the French Connection

[45B20]

I noted some questions as to the origins of the ¼”x25tpi thread size used on the front and rear trigger guard screws (or action screws as some call them) on the M1903 on the Gunsmithing Board.

This thread size is very old and is the last connection between the first musket built by the Springfield Armory, the M1795, and all (or at least most) of the muskets/rifles built by Springfield up to the M1903..

This ¼”x25tpi is the thread size used on the side plate screw of the French Model 1768 musket, commonly know as the ‘Charleville’, and the Springfield M1795 musket which “was virtually identical in all respects to the French Model 1768 musket“.

This ¼”x25tpi thread size was used by Springfield on all side plate screws up to and including the various ‘Trapdoor’ models. Then it was used to retain the action of the Krags and then finally the M1903s.

The Metric System was adopted in 1799, but the French, who were big promoters of the Metric System, started using metric thread forms before that. However the metric size of this thread size is M6.35x1 and I would think that if this thread size had metric origins its size would be M6x1 or M6.5x1. I suspect this thread size had its origins before the metric system was being developed. The thread size may have started as a M6.3x1 but Springfield may have made that M6.3 (0.248”) into ¼”. Not sure on this, just a guess.

I have noted that the 6.3x1 thread size is popular among clock makers so perhaps this thread size was in use years before the French Arsenal system started using it. The French then passed it on to the Springfield Armory via the Charleville.

I acquired the above information 20 or 30 years ago and I can not find my source material,,, so this is only from memory.


Years ago I had two taps marked 6.35x1 that I used to repair damaged threads on M1903 actions, but lately I only see the 6.3x1 size available. Since 6.3mm works out to 0.248” this is close enough for most repair work.

As Chuck pointed out, ¼”x25 taps are available thru Brownells, plug 748-001-425AA & bottom 748-002-425AA both are around $25. I do not believe that MSC Industrial Supply carries ¼”x25 taps any more, however they do list M6.3x1 taps, 77429439 at less than $10. If the guard screw hole in the receiver is really damaged you can first chase with an M6x1. Use a good lube, go slow and pray.

45B20

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[Chuckindenver]

great info.
to add. if you have rough threads, or other issues with guard screw holes. try and pick them clean first, before you tap, or force a screw in them...
the front screw hole blows out very easy...even easier on low number actions... alittle tougher on A3,s but still be careful.
remember, the 1903 has a shorter screw then the A3,...not much , but enough to blow the hole out on the rear tang screw, if your not careful,
also careful of forcing a 1917 screw into the hole...it will ruin your day, and likely destroy a good rifle.

[emmagee1917]

The wheel-locks were made by clock makers back in the day. The thread size prob'ly made the cross over at that time.
Chris

[45B20]

In my first post on the subject of M1903 guard Screws, I stated that this thread size came from the side plate screws of the ‘Charleville’ and then on to the U.S. M1795. I was wrong, the 1/4x25 came from the ‘Charleville’s tang Screw not the side plate screw. Makes a little more sense that a tang screw would evolve into a guard screw.

The side plate screws also have a thread form of 25 tpi, but the shank size being 0.197”- 0.198”.

The think that emmagee1917’s speculation that the 25tpi thread form came from the clock makers is about the most reasonable I have heard so far.

45B20

[Chuckindenver]

the 1917 was basicly a P14 made to fire a 30-06 round, since the P14 was made for the Brits, it was built for them threads and all, when they made the M1917 that was one thing they didnt change,
they even left the off set sling swivels for the long range sight on the P14..basicly changed what ever parts were needed to make the rifle fire 30-06.
however, more then you might think is different P14 VS 1917...but thats best left for another post, and has been discussed before.