Andy Wade coughed up some electrons that declared:

Dave Plowman (News) wrote:

Don't think that's so at all. There are very few sizes where AC will fit
Whitworth - and none I know of where AF will fit BSF. They were
essentially new threads to a common standard.

AC and AF? Do you mean American Coarse and Fine? I've not heard of
those designations before. Pre-WW2 American threads were NC and NF
(National Coarse/Fine) [1]. The unification which gave us UNC and UNF
[2] was, AIUI, not so much about making US parts interchangeable with
our BSW and BSF as about having a common standard adopted on both sides
of the pond. The Yanks would use UNC/UNF instead of NC/NF and we would
use UNC/UNF instead of BSW/BSF. This was only partially successful
because Whit-form parts remained widely available and used in the UK -
although the automotive industry did eventually switch to Unified.

[1] http://www.sizes.com/tools/thread_NCNF.htm
[2] http://www.sizes.com/tools/thread_american.htm


AF was quite common nomenclature about 20+ years ago, at least in the car
fixing circles...