Thanks David, that helps.
The nomenclature for BSP fittings is confusing. An 1/8" BSPP fitting, for example has a major thread diameter of 0.383", while a 1/4" BSP fitting has a major thread diameter of 0.50". The BSP number roughly corresponds to the ID of the pipe or fitting.
Maybe if you e-mail Merlin Motorsports they could solve confusion ? My memory is when I came across it web-surfing a couple of years ago is that I e-mailed or live chatted with them to make sure it was the proper one to fit my XK engine and a standard type mechanical gauge.
David
68 E-type FHC
The nomenclature for both BSP and the US NPT pipe fittings has a rich history. The idea, as you note, is to name the pipe diameter in reference to it’s ID. Early on, 1 inch pipe had a one inch ID. The OD was quite a lot bigger, due to beefy wall thickness. Thread pitch on the OD was standardized so that a given pipe could accept all of the common fittings–couplings, caps, tees, ells etc. Then, as is true for any manufactured item, they figured out that they could cut the wall thickness to save money. If they kept the ID (like 1 inch) that would mean that the OD would be smaller, and all of the fittings needed would have to be smaller too. So they kept the OD as standard and simply increased the ID. The different pipes with the same OD were referred to as schedule 40, schedule 80 etc. at least for NPT as it currently is sold. So 1/16, 1/8, 1/4, 3/8, 1/2, 3/4 etc pipe has an ID at least that size and usually larger, and an OD and thread pitch that has remained the same over history.
Copper, PVC and other plastic pipes have similar nomenclature, and various wall thicknesses that all use the same fittings.
Nerdy addendum: The BSP and NPT steel, brass and stainless threaded pipe sizes above were developed for water and their size is called “nominal” in reference to their ID, which varies. Copper pipe of particular nominal sizes happen to have precise ODs. For example, 1/2 inch nominal copper pipe is 5/8 OD. There are three “schedules”, type K, type L and type M. K has the thickest wall (smallest ID) and M the thinnest wall. But, unlike threaded pipe, the precise OD of these sizes allows a second system of reference. A plumber buys 1/2 inch copper pipe, which is 5/8 OD. But a tradesman in natural gas, HVAC, or compressed air buys 5/8 copper pipe of a particular wall thickness–reference is to OD, not nominal ID.
Just to round this out, when installing an English mechanical oil pressure gauge, the gauge will be 1/8 BSPP, with an internal gasket.
And, to make it really clear, I hope, or really confusing: There is BSPP and BSPT. The BSP in both is “B(ritish) S(tandard) P(ipe)”. The final P, if present, indicates “P(arallel)”, for a straight thread, while a final “T”, if present, indicates “T(apered)” thread, similar to the NPT pipes we in the US are familiar with.
I went ahead and ordered a 1/8 BSPP male to 1/4 BSPP female adaptor from McMaster-Carr. It arrived today. I screwed it into the filter head and can now verify the port in the filter head is definitely BSPP.