doublegunshop.com - home Forums General Discussion DIY Gunsmithing @ doublegunshop.com Gun thread types

I need to make up a lock screw for my trulock bros. side by side, as the threads are worn and stripped. Measuring the thread on the remainder of the screw, i get a diameter of 0.1695 inches and as far as my poor eyesight can tell, 36 threads per inch using a thread gauge. I want to buy a die to cut the threads.

But looking at the thread charts in my machinery's handbook, the only one i can see that comes close is no.8 UNF.

Would this be correct ? What threads systems were in use in the English gun trade about 1900 (which is roughly when i estimate the gun to have been made). Before i measured the thread i had B.A. threads in mind.

Were there specific gun threads ?

bonny,
The British used different thread forms and the dia./tpi is only a part of it. Some may be 60 deg. included angle like ours, some are 55 degrees Whitworth, some are flattened at the points and roots, and some pointed, then on some the tops and roots are rounded. If there are more than one lock screw, they are all likely the same. You may be able to get a better idea by checking an undamaged one. You may get more help from a model builders website. If you find the thread, it will likely be hard to duplicate the screw, because it is so short with the thread going to the head. If you use a lathe, I find it easier to turn the spindle by hand and watch under magnification( uncouple the motor and cutting a thousandths at a time, the spindle is easy to turn by hand. If you can find taps and a die, you can make a threaded rod and oversize round nut. Screw them together, heat the nut red hot and "forge weld" it to the threaded rod. Shorten to length, dress/size the nut, and cut a sacrificial slot. Screw it into the gun, mark the location of the final slot( to "clock" it), cut the slot and reduce head thickness to flush with the other screw and engrave to match. This gives a screw with a thread all the way to the head.
If you do it this way, you can recut the form to a known form in the action, With similar diameter and same pitch.
Mike

Bonny - grab yourself an M4.5 X .75 die. This will be close enough for this job. It's nominally about .180", but after you've threaded the shank in the lathe, flatten the thread crests off a little with a file, and run it in and out of the lock plate with a little oil. The plate should be hard, so the threads will iron down well.
This "swage fitting" will match the diameter/thread angles/ slightly different pitch up well enough. Don't worry about the thread angles on a job like this - you're not building a nuclear power plant.
The bloody Poms were all over the place with their threads. I have replaced a lot of screws with the above die - most matching perfectly.

Thank you Mike and Mike. I'll buy a M4.5 die and see how it goes.

A No. 3 BA screw has a no,I always dimension of 0.162� and a thread pitch of 34.8 TPI. Based on your reported measurements, the No.3 seems the closest fit. An M4-.7 has a TPI of 36.3 but the nominal diameter is only 0.157�.
I hope this is found to be useful.
Dave

Tap out to 4.5 mm close enough for gun work.