Bob, Zeto, thanks. And Ruairidh - thanks for finding this! David Cochrane recalled there was a thread (no pun intended!) on the topic but I had failed to find it.
For what it's worth:
The steel plugs I have taken out (3 brg engine) are definitely 28 tpi - yes it's hard to count accurately, but if you offer up to a 1/8 BSP tap under a magnifying glass the peaks coincide exactly.
As to whether they are parallel (BSPP) or tapered (BSPT), the taper angle of BSPT is 1:16, and it is harder still to judge whether the thread diameter drops off by 1/64" (0.4mm) over a quarter inch of worn thread. However, apart from a slight lead in on the first thread, I see no sign of a taper whatsoever (when offered up between the jaws of vernier calipers under a magnifying glass). My bet then is 1/8 BSPP.
Dave Mann made a valid point above - BSPP is not self-sealing and should normally be backed up with a ring seal, whereas BSPT tightens into the taper until it seals. I'm not sure this is significant given the oil pressure concerned and that any drips will only migrate back to the sump.
And just to re-iterate what I said above - I contacted one (Chinese) eBay seller to confirm whether their BSP screw was parallel or taper thread, and they replied oops sorry it's NPT, which as Bob rightly points out has a different thread angle. As others have said this may not be important - but buyer beware.
For what it's worth:
The steel plugs I have taken out (3 brg engine) are definitely 28 tpi - yes it's hard to count accurately, but if you offer up to a 1/8 BSP tap under a magnifying glass the peaks coincide exactly.
As to whether they are parallel (BSPP) or tapered (BSPT), the taper angle of BSPT is 1:16, and it is harder still to judge whether the thread diameter drops off by 1/64" (0.4mm) over a quarter inch of worn thread. However, apart from a slight lead in on the first thread, I see no sign of a taper whatsoever (when offered up between the jaws of vernier calipers under a magnifying glass). My bet then is 1/8 BSPP.
Dave Mann made a valid point above - BSPP is not self-sealing and should normally be backed up with a ring seal, whereas BSPT tightens into the taper until it seals. I'm not sure this is significant given the oil pressure concerned and that any drips will only migrate back to the sump.
And just to re-iterate what I said above - I contacted one (Chinese) eBay seller to confirm whether their BSP screw was parallel or taper thread, and they replied oops sorry it's NPT, which as Bob rightly points out has a different thread angle. As others have said this may not be important - but buyer beware.