Well I spent a happy hour or two in the garage measuring up plugs and ports and i was wrong about having BSP plugs and ports and Iain is correct
I discovered I had a mix of NPT and BSP plugs. When I sorted out and measured them carefully I found the plugs that looked original from engine blocks i had dismantled were all NPT in both the 1/4 and 1/8 sizes.
The ports in the engine blocks also look like they are NPT based on how the plugs thread in. Not possible to measure the inside thread pitch directly and the port diameters are identical for NPT and BSP so you cant tell by that either I discover. So how the plugs thread in is the only way to tell.
All the original 1/4 NPT plugs were around 9mm long and screwed in about 5 threads or 7mm to be about 2 mm above the surface before locking up on the taper. The 1/4 BSP plugs screwed in about the same distance until what felt like the threads jammed rather than the taper locking up
The 1/8 NPT plugs screwed in about 5 threads until the taper locked up, The 1/8 BSP plugs felt looser in the hole and screwed in about 8 threads until the taper locked up.
My conclusion is the threads in the block are NPT. You can screw BSP plugs into them in practice and get them to seal OK with appropriate thread sealant but you should use NPT. Incidently the water jacket drain tap or plug is also 1/4 NPT not BSP.
The people who decided to make NPT just slightly different from BSP sure created a challenge in identification.
cheers
Rohan