Quote Originally Posted by Steve Smith View Post
1/4" 20 threads per inch. Also known as 1/4" Whitworth (well... it is over here!).

Careful.... British Whitworth thread has a 55 degree "V", Most other threads have a 60 degree "V".

Many years ago I had a 650 BSA. All British Whitworth thread. So everytime something vibrated loose (and God did that thing vibrate) I couldn't just use any nut. I had to order a whitworth thread nut.

British Lee Enfield receivers have a 1"X14tpi receiver thread, but barrel threads must be cut with a 55 degree thread to mate properly with the receiver threads.

Sometimes the clearances are sloppy enough parts will fit even though the threads are dissimilar.

In North America, threads are usually designated as UNF or UNC now. (Unified National Fine, and Coarse) and have a 60 degree "V" with a small radius at the point.