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Thread angles


Well Known and Active Website User
VOC Member
In know thy beast, all thread angles are given except that for 20tpi threads. Could anyone advise me of the correct thread angle for these 20tpi threads. Thank's.:(

Tom Gaynor

Well Known and Active Website User
VOC Member
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.

Peter Stokes

Well Known and Active Website User
VOC Member
Whit. threads

On the subject of Whit. threads - there is an example of this familiar technology in very common modern use. The camera tripod socket is 1/4 Whit. Try measuring it on any (smallish) camera. Bigger cameras use a 3/8 Whit. thread, and it was common on middle-sized cameras with the bigger socket to find a little adaptor screwed in, so that a normal tripod could be used.

Are there other Imperial threads in common use in new equipment?


Robert Watson

Well Known and Active Website User
VOC Member
Have you measured the thread angle, or are you assuming because it is 1/4 - 20 tpi it is Whit, as UNC is also 1/4 - 20 tpi, and quite frankly I'm not sure a bolt would notice whether the threads were 55 or 60 degree angle!!

Peter Stokes

Well Known and Active Website User
VOC Member
Whit. or not?

Hello Robert.

You may be right, and the threads are usually quite 'loose' to allow easy putting together without cross threading. I have always understood them to be Whit. though - I think I have some old notes somewhere stating it.


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