How do i measure thread of this tank valve? (seems less intuitive than I imaged)

[Brian Pasic]

Using a piece of string and a ruler how would i measure the thread of this tank valve.
Is it top to bottom or completely around. I know it seems like a simple question but measuring thread seems less intuitive than I imagined. Also is measuring this thread the same as measuring the thread on a scuba pressure gauge hose or 2nd stage regulator hose?

regards

107046

[KWS]

MEASURING THE TRHEAD IS the number of threads per inch. measure top to bottom of the threaded and count the threads. math it out to get 14 threads per inch. if you want to get the size you can measure across and measure 3/4 7/8 inch or what ever. most valves are standard size.

[Bob DBF]

With the 3000# service it is most likley a 3/4" parallel pipe thread. The 7/8 thread used for DIN 300 Bar tanks is a machine thread standard and is smaller in diameter than the 3/4" pipe. A 1/2" tapered pipe thread was also used but you dont see them much except on vintage gear, along with other semi home brew configurations.

How come?



Bob
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I may be old, but I’m not dead yet.

[Scared Silly]

The valve is a 3/4" - 14 NPSM thread.

Measuring the thread diameter is fairly standard (as described above), what is not standard, is the standard.

Here are the two most common straight thread scuba valves (there is also a 1/2" - 14 NPSM)

3/4" - 14 NPSM
7/8" - 14 UNF

The 7/8" UNF is actually smaller than the 3/4" NPSM when measured, further 3/4" is greater than 3/4" that is because the NPSM includes the outer thickness whereas the UNF is the actual thread diameter.

[Akimbo]

3/4" - 14 NPSM, the most common size in the US, actually measure about 1.034" at the major diameter of the male thread. Note that there are also several European standards, 25mm being the most common. Unfortunately the diameter is fairly close (without a caliper or micrometer) to our 3/4" - 14 NPSM so count the threads/inch to make sure.

On the other hand, you could just take it to any dive shop and see if the threads mesh to another valve. The dip-tube may have to be removed from both valves.

[captain]

Two different engineering standards are used for valve thread, US straight pipe thread and ISO metric threads.

Threads - Metric ISO 724 (http://www.engineeringtoolbox.com/metric-threads-d_777.html)

American Standard Straight Pipe (http://www.engineeringtoolbox.com/american-standard-straight-pipe-threads-d_1520.html)