Another interesting tidbit:. Whitworth was used for larger screws, but this system was made for screws of 6mm diameter down to the tiniest watch screw.

The BA thread is a metric based system devised when the prediction was SAE would be extinct within 100 years. The threads are 47.5 degrees included angle. Though obsolete for the MOST part, there are a smattering of niche uses and I found a British firm that carries that tap. Six-gun and gunny: I ap...

After exhaustive searching and measuring, I discovered that the thread is 4 BA, which is an obsolete British thread. It is 38.46 tpi. Yep, strange and interesting.

I am trying to re create the loading gate screw on a WK 44-40, but it is giving me some fits. Can anyone check the thread count? I figured it's an oversized 6-40 but having trouble there.