Correct thread pitch for the 68 era petcocks?

[gtsun]

My 68 for 441 shooting star has those brass valve type petcocks. Some people say it's quarter inch NPT but I've seen others say they are quarter inch BSP. Which one is correct and what's the difference?

 

[L.A.B.]

My 68 for 441 shooting star has those brass valve type petcocks. Some people say it's quarter inch NPT but I've seen others say they are quarter inch BSP. Which one is correct and what's the difference?

= 1/4" BSP(P) (or '1/4 Gas') which is 0.518" x 19 TPI, 55-degree parallel thread.

BSP
http://www.britishfasteners.com/threads/bsp.html

NPT is a tapered thread. The nearest US equivalent to BSP would be parallel NPS(M) however, thread angle (60 deg.) nominal diameter and TPI (0.540" x 18) are all different to BSPP (however, BSA and Triumph oil pressure switches were 1/8" NPT (early) and 1/8" NPS (later).

NPT
http://www.gewinde-normen.de/en/npt-pipe-thread.html
NPS
http://www.gewinde-normen.de/en/npsm-pipe-thread.html

 

[gtsun]

Thanks L.A.B. When I took mine out they are actually 1/8" BSP. Not 1/4". Perhaps the later BSA's had 1/4".

 

[L.A.B.]

When I took mine out they are actually 1/8" BSP. Not 1/4". Perhaps the later BSA's had 1/4".

1/4 BSP was the usual size from around 1970. Before then, I believe they did use other sizes, 1/8", 3/8" etc. just to keep things interesting! :wink:

 

[Bob Z.]

If your tank uses the larger 3/8" size fuel tap this adapter is nice and let's you use a wider range of 1/4" taps.
https://www.eurojamb.com/petcock-adapto ... csn3e9plv6