Tank Valve Threads

[Lemonade]

There are two commonly used valve thread sizes:
"3/4 - 14 NPSM" and "7/8 - 14 UNF"

As far as I know, former is commonly used in Aluminum and LP Steel tanks, latter - in HP Steel.

:question:
Does anyone know what those numbers and letters actually mean and represent?

 

[Scubaroo]

NPSM is "National Standard Free-Fitting Straight Mechanical Pipe Thread"

UNF is "Unified Fine Thread" (I think)

The 3/4 and 7/8 refers to the diameter of the thread - this could be either the diameter to the top or bottom of the threads, not sure what the US standards are. The 14 is most likely the number of threads per inch.

Note that there are some metric tank valves floating around, especially in Europe - I read somewhere (can dig it up if anyone needs to know) that a tank rupture that resulted in a death was attributed to a metric-threaded tank valve being used in an imperial-threaded tank. So no buying cheap tank valves or manifolds from Simply Scuba if you live in the US!!!