"The Enfield inch" when an inch is not an inch!

[Bruce_in_Oz]

And then there is the "utterly Imperial" BA system.

Well, sort of.

It is almost a direct lift from the old Swiss "Thury" system used by yodelling clock-makers. Start with "0" BA. It has a pitch of, give or take a gnats knacker, 1mm, just like M6. The form and flank angle (47.5deg) are, of course, completely different.

"1" BA is simply "0" BA multiplied by 0.9 to give to the next smallest thread. This process repeats ad infinitum (ad nauseum?).

Hence the almost incomprehensible "numbers" attendant to BA threads. I have NEVER even attempted to cut one on a lathe; metric or otherwise. just keep a set of BA taps and dies in the toybox.

An example of the "almost" relationship is the rear-sight adjustment screws on the FAL (metric)/ L1A1 (BA); VERY similar in diameter AND pitch but not close enough.

In the process of "Imperializing" the FAL to arrive at the L1A1, screws of the nearest "industry-standard" appear to have been sought, be they BA, BSF or something different like a Unified number thread or somesuch.

Remember these were the days when Vauxhall (GM UK) built engines with bolts having UNC threads but Whitworth-sized heads. Detroit-designed motors meant to be worked on by UK mechanics with "UK standard" tools.