I have a valve as below, looks like 1/4
BSP / NPT Pipe thread sizes are strange to the unintiated... they have NOTHING to do with the outside diameter...rather the size applies to the nominal pipe bore.

1/4 BSP / NPT would be ...1/4ish bore and 1/2 O.D.!

Also BSP and NPT are very similar but the Americans deliberately made their threads slightly different....

http://www.teamonslaught.fsnet.co.uk/threads.htm