Thread Size

[jim14th]

Happen to know the tap & die and or the thread size for the rear sight base screw on a Model 61-64 Springfield musket..thanks

[hawkeye2]

10-32, very shallow hole

[John Bly]

I'm pretty sure it's an #8-32 thread. At least that is what I've used and it worked. A #8 screw is .164" diameter. A #10 is .190" diameter. (formula for # size screws = Number times 13 plus 60 divided by 1000)

[geezmo]

John is right, 8-32. I tried an original sight screw in my Brownell's "Thread Checkr" this morning and it fit like a glove. I've often wondered about the relationship between size number and actual diameter but could never find it, even on the internet. Now with that formula if I know one I can solve for the other.

John, thanks a lot for posting that.

Barry S.

[Muley Gil]

And who said we would never need algebra when we grew up! :D

[RaiderANV]

I thought that about trigonometry until I got into construction.

[geezmo]

Who'd a thunk it?? Glad I got me book learnin'.

[jim14th]

Thanks for the Info and input..jim

[Kyle G.]

I have an original 1863 model '61 special Colt rifle musket that had the rear sight screw hole drilled and threaded crooked. This resulted in a loose rear sight base that wouldn't seat into the dovetail and the sight leaf never sat flush with the sight base. A loose rear sight isn't very conducive to accuracy so I brazed the hole closed, filed it flat, and re-drilled the hole in the proper location on my series I Bridgeport to keep the sight base screwed down tight. My original screw had been mangled from me trying to thread it in deeper in a crooked hole with a punch. I ordered an original replacement screw from Dixie Gun Works and had the pleasant surprise to find the thread was #8-30 after close examination with several different thread gauges. A special order for a #8-30 bottoming tap cleared that up and now the rear sight base is screwed down tight and the sight leaf lays down flush like its supposed to.