First, you state, " John confirmed that he believes they are BSP as that is what Yanmar typically uses."
BSP (British Standard Pipe) is simply a set of thread standards. You need to know whether it is BSPP (British Standard Pipe Parallel) or BSPT (British Standard Pipe Tapered if you want a correct fit. What I've seen is that NPS is considered to be an acronym for National Pipe Standard when it is National Pipe STRAIGHT (all threads the same.). The British use the term PARALLEL to define all threads being the same. Tapered is tapered, where female threads start with a thread or two equal to the straight thread dimensions and then taper down and the male threads start small and taper up to the straight thread dimensions. All John is confirming is that it may be a BSP thread.
NPT: National Pipe Taper correspond to BSPT
NPS: National Pipe Straight correspond to BSPP
You stated, "My understanding is that with BSP, the male thread is tapered and the female parallel."
The answer to this is NO. BSPP (parallel) and BSPT (tapered) threads are not compatible and the same would be true for NPT and NPS threads.
And, YES the male BSPT fitting will thread into the BSPP female fitting. The reverse will not work. However, the male thread will have most likely only one or two BSPP threads at the back end of the fitting. Neither a good sealing option or thread purchase
option, but it will work in a pinch.
Here's a thought, on the old Yanmar fitting/valve that screws into the engine block, was there a gasket
or O-ring as part of the fitting? Or is there an internal shelf that will stop the male fitting. Or did you simply use some paste or tape on the threads and then screw it in till it stopped turning. If there is no shelf or O-ring/gasket as part of the original Yanmar valve/fitting assembly then the threads are most likely tapered. Most pipe threads are tapered if they have no compression gasket
or o-ring. I very much doubt that Yanmar would use a female BSPP thread on the engine and a male BSPT thread on the valve/fitting. Just simply not good engineering.
Here’s some additional food
for thought info.
I'm not an expert on BSP threads, but do know that the differences between NP and BSP do not effect the thread type, tapered or parallel. In other words, NPS (National Pipe Straight and BSPP (British Standard Pipe Parallel) have the same basic configuration and function. They all screw in till they bottom out, and their sealing quality is by a bottom or top compression
, usually with an O ring or gasket. They are basically the same as the threads on a nut and bolt.
If you look at an NPS and BSPP set of threads you'll see that every thread is the same. If you put a caliper on them both the internal fitting surface and the thread peak surface would be parallel.
I call these threads "sloppy" when it comes to sealing. Again, all the sealing is done by top and bottom compression.
NPT (National Pipe Tapered) and BSPT (British Standard Pipe Tapered) are designed to seal, not by compression, but by the meshing of the threads with one another (with the help of a little paste or tape. Although, given the low pressures involved on most thru-hull related applications a paste or tape on a parallel thread will suffice.
A tapered thread is designed to thread in only so many turns before maximum sealing occurs. And should require only minimum sealant to achieve reasonably high pressure sealing. In fact, there is a form of tapered threads NPTF (National Pipe Tapered Fine) that is so close tolerance machined that it will seal with no sealant.
An NPT 1/2" tapered thread according to the ASTM standard should have 14 threads/inch and have a hand-tight engagement of 4.48 turns. The BSPT 1/2" threads will have a similar, but slightly different, set of specifications. In theory, you should get maximum sealing at a point just over the number of hand tight turns.
The BSPP female thread needs to accept 6.48 threads to get any real mechanical purchase
; the 4.48 threads with a 3.20 inch length – the are in effect doing very little with regard to either mechanical purchase or sealing.