Oil Sending Unit Question

Can anybody tell me the thread dimensions on the Series II oil pressure sending unit? I’m trying to determine if it will fit into this threaded hole, where the red arrow is pointing without any kind of an adaptor.

Screenshot 2021-01-12 134020

Photo by Phil Dobson.

I believe it’s 1/8 BSPP.

Thanks Ray. …

1/4 inch BSPP. The hole shown, if for the cam feed on a S3 oil filter block, is I think 1/8 BSPT. It takes an adaptor that accepts a flare fitting for 1/4 inch steel tubing. It may be the same adaptor that connects the EGR pipes to the exhaust side of the head.

Thanks Robert. I just went down and closely examined the fitting that came out of the filter head. It does appear to have a very slight taper and measures to about .38" at the threads which is 1/8" in British Pipe sizing.


Note that BSPP and BSPT can be persuaded to go into the same fitting, but the threads will get buggered. Similarly, 1/8 NPT is close enough to go in, with a bit more effort. So, what you have may be somewhat damaged, but I am about 99% certain 1/8 BSPP is correct. The copper/aluminum washer provides the seal, not a tapered thread.

You are talking about the threads of the OP gauge sender (as John inquired about), Ray? Aren’t you making up some senders for your new stepper motor gauges? Is that the thread you are using, not 1/4 BSPP?


Pretty sure, but it’s been forever since I’ve looked at the senders. I picked out the ones I’m using years ago. I’m heading out to the shop a little later, so I can verify it.

Well, this is what old age does to you. Last time I had the oil filter block off my engine was probably 15 years ago, when I replaced the Smith’s sender with a VDO. At the same time, I think I put a 1/4 BSPP to-1/8 NPT reducer in the filter block, because that’s what the VDO sender had, and 1/8NPT is what the solid state sender I’ve used for the last few years has. I believe the thread in the filter block itself is 1/4 BSPP, as Clive said.

Correct. 1/4" BSPP in the filter head for a Series 2.

Is the sender itself perhaps 1/8 BSPP, with a factory-installed adaptor? I don’t recall adding a 1/4-1/8 adaptor when I first installed the VDO sender, and the one installed in my filter block does not look like any I would’ve been able to source locally (it’s plated steel, rather the the far more readily available solid brass parts, and looks really old).

1/4 BSPP, One of those rare non-SAE threads.

BSPP Fittings | British Standard Pipe | Hydraulics Direct

The hole in the Series III XJ filter block I posted a picture of takes a fitting that is 1/8" BSP, it appears to be very slightly tapered, verified by the fact it’s progressively harder to to turn as it’s screwed into the filter head.

Given the measurement in the below picture it’s got to be 1/8" BSPT.

So if the stock oil pressure sender for a Series II E-type is 1/4" BSPP I would need an adaptor fitting.

Is it the same for S1? Is the sender 1/4 or 1/8?

Beats me. …

Here is the tee piece I got that screws into the filter block and also fits the sending unit on my '68. Does it tell you what you are asking in the description ?

68 E-type FHC

Not really. That is clearly a 1/4-1/4 fitting. Does your sender screw directly into that, with NO reducer?

All of the Smiths bimetallic senders (for either 60 PSI or 80 PSI) two terminal bimetallic gauges, pre-S3 XJ6-12 were AFAIK 1/4 BSPP. All of the various OP switches were 1/8 BSPT, fitted into the intake side 1/2-20 TPI main bearing drillings. The XJ6 S3 filter blocks fed the cams from a tapped hole located approximately where the OP sender had been located previously. I think (not sure) that the S3 gauges were 3-wire (not requiring voltage regulation) and a different (resistive) OP sender was used. Senders were fed from various adapters screwed into the main drillings. IMHO from senescent memory.

Yes that tee adapter screws into my filter block and and my original sending unit screws into it with no type of reducer. I did buy copper crush washers to use with it. Here is my set-up using that tee when I need to check accuracy of the stock gauge/sending unit.

68 E-type FHC