Early 19th Century French screw sizes

[eddillon]

Building a Boutet style pistol.  Fortunate enough to have an original French lock.  lock bolt hole was already drilled.  Location is perfect in relation to architecture of the build.  Lock plate is quite hard and I'm sure it would eat up a tap if I tried to convert to an 8-32 bolt.  Minor diameter of the hole is .2 inches or 5.08 mm.  Thread doesn't appear to be very fine.  I thought I would try a 6-32 or 6-28.  Lock has never been on a gun.  Frizzen never struck.  Picture better than a thousand words.  Any ideas out there?? Correction: .120



Close up of the lock bolt hole.  BTW those are shadows around the lock not gaps.  Picture taken when there was about .060" to go.

[Jim Kibler]

That's a really big minor diameter.  Are you sure this is correct?  As far as the pitch goes, you can often screw in a piece of wood and screw it back out.  this will leave an imprint and then you can deduce what the threads per inch are.  Another way is to just try some different screws or taps and see if you find any that work.

[Clark Badgett]

The French were using metric by then, they invented it. It could be an M6x1 hole, which would have a minor diameter close to what you measured.

Or it could be a size long forgotten.

[Acer Saccharum]

Wrap a little Silly Putty around a toothpick, and press it against the sidewall of the threaded hole. Pull it out carefully without distorting it, and measure with a thread pitch gage.

Once the pitch is determined, then a screw can be made on a lathe smaller and smaller until it fits.

[jerrywh]

If you have any cerrosafe you can melt it into the hole, cut a screw slot in it and screw it out. the cast will tell you exactly what you have.  You only need to know the pitch so even a hardwood or hard wax would work.
  I had a pair of original French locks at one time and Nothing was the same . Not even any modern metric would work. I guess they had their own set of taps and dies.

[Bob Roller]

Go to a NAPA car parts store and see if they have any 5MM bolts and try one in the lock plate and see what happens.

Bob Roller

[Bob Roller]

When you put that lock back together would you show us a picture of the mechanism?
 Bob Roller

[eddillon]

Bob,
Will be inletting the innards of the lock tomorrow.  Should post a photo of the working side after that.  Next pistol will have your little Manton on it!  Same style but with bronze barrel.
Cheers,
Ed

[Bob Roller]

Ed,
Sounds like a plan to me.Externally the French lock is beautiful but I wonder how well it would stand up to a lot of shooting.It seems to me these locks and guns survive in fine condition because of little to no real use.Anyhow,we'd like to see the "engine" in this lock.

Bob Roller

[eddillon]

An update:
Lock is all inletted.  The lock bolt thread has a minor diam. of .120 and a major diam. of .142.  As near as I can figure the pitch seems to be 44-48 tpi.  Is there something close out there?  There is only one lock bolt.  The front of the lock hooks under a screw head.  Internal picture for Bob Roller.  I'll post a photo of the fully assembled lock later, Bob.  BTW, flash pan lined with platinum or white gold.

[davec2]

Is the lock plate hardened?

[eddillon]

Is the lock plate hardened?

It will dull the best of files with no mark on the lock!

[Bob Roller]

Ed,
Many thanks for posting the picture.That is an elegant little lock and rare as a football bat off of a gun.I along with others will await the posting of the complete lock.

Bob Roller

[James Wilson Everett]

I often make screws for the original locks where the screw threads are unique.  I recently made two such screws for an original gunlock under the "shade tree' at Schoenbrunn, Ohio with no electricity or modern tooling at all.  Not really a difficult job if you use 18th c tools and materials, but probably a lot more difficult with modern machines. Let me know, maybe we could make this into a tutorial of HDTDT of making odd sized screw threads.

Jim

[eddillon]

I often make screws for the original locks where the screw threads are unique.  I recently made two such screws for an original gunlock under the "shade tree' at Schoenbrunn, Ohio with no electricity or modern tooling at all.  Not really a difficult job if you use 18th c tools and materials, but probably a lot more difficult with modern machines. Let me know, maybe we could make this into a tutorial of HDTDT of making odd sized screw threads.

Jim

Jim,
This sounds very interesting.  I would welcome a tutorial.

[Dr. Tim-Boone]

Ed would you also take a couple of pictures to help us see how the hook of the front of the lock hooks over a screw head?
Thanks

[PPatch]

That little lock is a beauty; the frizzen spring is a work of art. Please post photos when the pistol is finished. I too would like to see pictures of the hook. I have seen other examples but that one looks different and I have a front lock hook to do on my present project.

Thank you Ed.

dave

[eddillon]

For Dr. Boone and PPatch

Ed would you also take a couple of pictures to help us see how the hook of the front of the lock hooks over a screw head?
Thanks

[Bob Roller]

Ed,
I referenced the diameter into metric screws and a 3.5x.60mm seems to be the major diameter that you described.That is one size I don't have and I'd make up a screw for you to try.

Bob Roller