Right Angled Steam Valve  - Half inch pictured

These valves, like the whistle valves have 1/2" NPT threads. The only difference between NPT and BSP is the thread angle, a difference of 5'

What we have found with the whistle valves is that they screw onto BSP threads tightly and without leakage. Alternatively you can run a tap up

the thread to remove some metal but then we find you need sealant of some sort.