On Monday, July 28, 2014 9:35:10 PM UTC+1, Onetap wrote:
On Saturday, July 26, 2014 6:24:05 AM UTC+1, harry wrote:

The union threads are parallel, but the pipe threads by which the 2 parts of the union are fixed to the pipe are usually BSP taper.



Some unions have flat faces and require a washer to seal.



Some (Navy pattern) have a spherical bronze seat and don't need a washer.

Picture;
http://en.wikipedia.org/wiki/Piping_...ubung_4072.jpg

Lots of other types of unions, though.