Bridal Screw Hole Tap size

[arttodd3]

Does anyone know the tap size for the bridal screw threads in the lock plate? I actually have to chase the threads in one hole in an 1842 lock plate but it is same size and thread as the1873 lock plate. In addition, I will need to get a tap so does anyone know of a source? I'm assuming the pitch is non-standard but I may be wrong. I do not presently have a thread gauge available.

Thanks in advance for any help anyone might be able to provide!

[John S.]

I think that the M1842 lock screws are different thread size than the M1855-1888.
Not 100% sure, but better check that to be sure.

[arttodd3]

I'll double check, John, but I ran a screw into both lock plates in the other bridle screw holes and it went in smoothly. If it isn't the same, it is awfully close.

[Dick Hosmer]

Nearly all (all?) SA screws are 26tpi. Al had, at one time, all the dope, and at least some of the taps and dies, as well as the sources (think British bicycle shops!) for them. I've always suspected that 26 was as close as we could (or cared to) come to 25.4, when we were copying almost everything from the French. Al could give you the definitive answer, I'm sure.

[arttodd3]

Thanks, Dick!

[Tony Beck]

#10-26 and #12-26 are the two sizes you need for lock and tang screws. They are not exactly right, but close enough to work. The original Springfield thread used a 55 degree angle with a circular root and tip profile (essentially the Whitworth thread form). Modern threads are 60 degrees with a square root. There are a couple places that list taps and dies. The taps are not expensive. Dies are pricier. I will check my notes, but you should be able to find them with an internet search.

[arttodd3]

Thanks a bunch, Tony. That really helps me a lot! I should be able to find a tap without too much trouble.