Roger wrote:

Half inch bsp is 14 turns per inch. My source confusingly quotes the
dimensions in metric:

The threads formerly known as BSP are now officially metric threads: BS
21:1985 and EN 10226-1:2004 give the dimensions in metric units. Many
dimensions may just happen to be simple multiples or sub-multiples of
25.4, but that's just a coincidence ;~)

--
Andy