Here's one I made for my 2½" gauge Helen Long:


If I remember correctly, the thread on that is 3/8" Whit. The seat is bearing grade PEEK which contains PTFE and Graphite. The spring was supposed to take up any backlash in the thread but proved to be unnecessary,
I made a couple of smaller ones for two Tich's that used 5/16" Whit threads.
As Fizzy says though, ball (gas) valves are very good if you have the room to fit one. Just make sure it is rated for the pressure and the temperature.
John
Edited By John Baguley on 27/03/2015 00:36:50