English or metric. How to tell?

[Bruce_in_Oz]

Yep! That Zero BA diameter detail seems to have missed "publication" in my first post.

The other very British" thing about BA threads is the size relationship:

To get the dimensions of a size say, 1 BA, multiply the previous larger size (Zero) buy 0.9. Ditto all the way down.

Had to be something to do with manual gearbox lathes that could be adjusted in those multiples. For us mature chaps, the original Meccano screws and nuts were all BA, Apparently, when the French took over, they changed to the nearest metric thread.

And then, there are "Enfield" screws; bearing no resemblance to any other on the planet. From the Martini Henry to the last No1 Mk 6, and including the P13 / P14, all were held together with "Enfield Specials", except the Lee Enfield stock bolt, which is 7/16" Whitworth. Then, there are the VERY British "Admiralty" threads, as used by the entire Japanese civil and military ship-building industry until fairly recently.

Mausers up to the M98 and the Type 30 and 38 Arisakas are full of Imperial threads, not so much the Type 99.

There is also a range of "aerospace / aeronautical" screws that have a simi8lar "form" to BA; apparently a good thing when bolting together components made from Aluminium alloys.