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

Pitch 1.814
Major diameter 20.455
Minor diameter 18.631 (0.73")

--
Roger Chapman

Thanks for the help.
Usuing a high tech ruler across the ends your metric measurements
appear to match my sample. Half inch bsp it is then. I wonder where the
half inch size comes from....

Don't forget to check the pitch as well. Not likely to be anything else
but ...

The half inch is the bore of the thick walled iron pipe the male threads
are usually to be found on.

--
Roger Chapman