Oil pressure gauge switch thread

[RustAndMagic]

So I tried to figure out what thread the oil pressure switch is using but I have not found any 100% answer so far.

Some say it's BSP(T) or actually BSPP I think (would require washer), some say it's NPT, EMS's ACC203 say it is NPT in all holes, but again, many say the engine block is BSP/BSPP.

Some say it "might" be BSP but it's ok to put an NPT sensor/adapter there (I don't agree).

Brave's AI say it's BSP or NPT

Anyone know 100% sure what it is ? is it different between years or are they all the blocks same ?

I tried to find anything in the workshop manual but have not located anyting yet.

(Question is about a 1958, 948cc engine)

[svenedin]

The thread is 1/8" x 28 TPI BSP. Early switches are parallel thread and need a washer to seal. All the modern switches seem to be taper thread. They do not screw all the way in and do not need a washer to seal.

Stephen

[philthehill]

I have measured the threads in the block and on the switch with both a tap and die nut and I find that they are 1/8" x 27 tpi NPT.
If you have a taper thread switch or adapter they will both screw into either a NPT or BSP parallel thread.
The taper thread does not go fully home into the female parallel thread. Do not overtighten as there is a possibility that the tread will shear if you do.
The majority of threads and their sizes are shown in the Mechanical section under 'Thread Sizes'.

Oil pressure 'T' piece.jpg (95.69 KiB) Viewed 2082 times

[moggiethouable]

Just as a generalisation.
Npt and Bsp threads have a different crown, that is the form of the thread at its peak or top is different.
A thread called a "uni-thread" was invented in the 1980's that accommodated both types giving a form of interchangeability, it did not catch on as it wobbled its way in both bsp and npt female threads, utilising a copper washer to seal.
It just did not feel right.
Any fitting needs to screw in more than the 1-2 threads you will achieve if you try to marry npt with bsp.
Having ascertained the correct thread you will find the female is parallel, a taper thread male with a little thread tape will do the job.
A sealing washer will not be necessary with taper male to female parallel, your female thread will not be taper, it will be parallel.
I fitted an oil pressure gauge kit with no real issues, it came from ESM complete with fittings.
I assume this is your intention.