NPT & BSP Question

[john2k]

I have a ceiling mounted shower arm that comes with a G1/2" connection for shower head and the other end is a NPT male 1/2" but is supplied with a BSP 1/2" male to NPT 1/2" female adaptor. But it does not mention which side of the arm itself is the NPT side and which side is the G1/2" side. Is there a way to determine this? I tried reaching out to seller but no reply. One end has more number of threads than the other side (not sure if that helps)

Thanks

 

[oldbuffer]

1. Both threads (1/2" BSP and 1/2" NPT) are 14 threads per inch (TPI)
2. NPT threads are 60 degree included angle, whereas BSP are 55 degree (i.e. Whitworth form)
3. The easiest way to determine which is which, without a thread gauge, is to try a 1/2" BSP female thread onto both ends of the adaptor. Whichever end is the easier (and possibly only depending on how well the threads are made) fit is the BSP end.
4. A 1/2" BSP basin back nut (e.g. Screwfix item No. 66268) would be a reasonable gauge. However, most compression nuts from 15 mm fittings (e.g. isolation valves, couplers, elbow) are also 1/2" BSP.