Petcock thread pitch

[tahoethumper]

Hey all, did a little research in the archives, and of course used a thread pitch gauge up against the brass petcock threads on my chrome tank, and came to the conclusion that the threads are 1/8-27. Just wanted to confirm this with others before I use the wrong tap. Thanks!

(I did notice the 28 npt was very similar)

[Dale]

1/8-28 BSPP

[go_hercules]

Yes, the correct thread is 3/8-28. This is a BSPP (British Standard Pipe Parallel). There is also a BSPT in that thread, which is British Standard Pipe Taper, so be careful. This thread is now referred to as a G1/8 rather than a BSPP, but you will see it called both. Don't let the 3/8 versus 1/8 thing throw you, because pipe is sized by the internal diameter hence the 1/8 reference. The outside major diameter of the thread is in fact 3/8. The thread form angle on SAE thread that we're used to is 60 degrees. The G1/8 on the other hand used a 55 degree thread. Do not use 3/8-27, which is commonly used on lamp bases. It is close but not correct. I have cut a number of G1/8 threads with taps, dies, and on single-point on the lathe. G1/8 taps and dies are not easy to come by but you can usually find a few listed on fleabay. The G1/8 (BSPP) thread was common on British petcocks in the day which I guess is why Hodaka chose that thread rather than metric. Hope that helps.