Thread flank angles
Trevor is correct, of course. But........
(Bear in mind that I have lived in Aberdeen for 30 years. Copper wire was discovered here when two Aberdonians found a penny at the same instant, and was where a horse stood on a sixpence in Union Street, and had to be shot. My father pointed out, more to the point, that Aberdeen was serving spirits in English short-measure 1/5 gills before WW II, when the civilised world (the rest of Scotland) served "quarters". This is a town where The City's idea of "financial prudence" (lending money to people who can't pay) looks verrry much like reckless profliglacy in the hope that someone else'll have to pay......"
Some ANF / UNF (same thing) threads, although 60 degrees, work very well in CEI (co****) 55 degree 20 tpi threads. 7/16" and 1/2" UNF (but check) are two. Alas 9/16" x 20 CEI has no UNF equivalent. Why UNF? UNF taps are not only cheap as chips, but are usually made by reputable US and UK manufacturers. Third World 20 tpi CEI taps / dies may indeed be 55 degrees (or may not) but they ain't cheap.