There are two different BSP threads. BSPP which is a parallel thread(like a bolt) and BSPT which is a tapered thread(pipe fitting). The flanges on the engine should be BSPT. The issue is 1.5" BSPT is 11 threads per inch where 1.5" NPT is 11.5 threads per inch. So, after a couple of threads of engagement it takes up any slack in the thread form and binds. There is also a difference in thread pitch and form between BSPT and NPT. One cannot simply rethread BSPT thread to NPT. You either need to have an adapter fitting(one end BSPT and the other NPT) or run the whole pipe system in BSPT.
You can also try old school pipe sealing cord/thread. Below is a link.
https://www.amazon.com/Loctite-442-35082