Help identifying thread

[Jake M]

As much as I love machining, I’m not a car guy nor an engine guy (automotive work is third on my list below plumbing and painting in my pantheon of hated activities).

I thought: What is an “oil pressure sender”, how does it work, and what does it look like?

It never I occurred to me until this moment that the oil pressure gauge in my cars needed some sort of sensor…

Google has been slightly helpful, but I’m still a little unclear.

They come in all sizes. A metal bulb, sometimes as a bulb or dome. Some shape that's easy (cheap) to stamp, and sometimes the bulb is small enough to fit into a small hex on the fitting. Threads on one end, and a one, two, or three wire connector depending on the design of the circuitry. With many types of pressure to resistance conversions inside. Or some land rovers (although in my limited knowledge, I think it's older than the one here), they do some crazy ass chit with external resistors, buffers, an internal resistor, and a bimetal spring, and use the cycling of that spring, through the buffers and resisters to drive a gauge.... Something like that. It'll mess with your head if you ever threaten one of those with a volt meter....
Typically when "car guys" talk about a "sender", what they actually meant to say was "sending unit". That will have an analog response, appropriate for driving an analog gauge. An oil pressure switch (or just "oil switch") is the same thing, but it just opens or closes a circuit at a specified pressure. Depending on if you have an oil pressure gauge or just a light. And on anything new enough to not be a hobby.... There tiny, it's just a pressure transducer and the ECM monitors, interpolates, and broadcasts the calculated oil pressure and most of the other engine data on the network, for all the other modules, including the cluster, to pick and choose what they want from it, and display it in whatever gauge format you bought. (or selected from the menu...). The one Hobby2409 has (I believe) is an inch and a half metal "can" with threads to screw into an oil port on the engine, and a single quick connect square spade terminal on the top, with a simple "variable resistance to ground" signal.

 

[Rex Walters]

They come in all sizes. A metal bulb, sometimes as a bulb or dome. Some shape that's easy (cheap) to stamp, and sometimes the bulb is small enough to fit into a small hex on the fitting. Threads on one end, and a one, two, or three wire connector depending on the design of the circuitry. With many types of pressure to resistance conversions inside. Or some land rovers (although in my limited knowledge, I think it's older than the one here), they do some crazy ass chit with external resistors, buffers, an internal resistor, and a bimetal spring, and use the cycling of that spring, through the buffers and resisters to drive a gauge.... Something like that. It'll mess with your head if you ever threaten one of those with a volt meter....
Typically when "car guys" talk about a "sender", what they actually meant to say was "sending unit". That will have an analog response, appropriate for driving an analog gauge. An oil pressure switch (or just "oil switch") is the same thing, but it just opens or closes a circuit at a specified pressure. Depending on if you have an oil pressure gauge or just a light. And on anything new enough to not be a hobby.... There tiny, it's just a pressure transducer and the ECM monitors, interpolates, and broadcasts the calculated oil pressure and most of the other engine data on the network, for all the other modules, including the cluster, to pick and choose what they want from it, and display it in whatever gauge format you bought. (or selected from the menu...). The one Hobby2409 has (I believe) is an inch and a half metal "can" with threads to screw into an oil port on the engine, and a single quick connect square spade terminal on the top, with a simple "variable resistance to ground" signal.

Perfect. Thank you!

Some sort of pressure transducer that varies wildly by manufacturer, model and year.

Good enough for me! Back to ignoring cars…

 

[whitmore]

If it's for a British car then it's probably British standard parallel pipe thread (BSPP) and probably either 1/8" or 1/4" size

Close to 20 tpi, BSP has 19 threads/inch, and NPTF (fuel variant of US standard) has 18 threads/inch. Twenty is... an
oddball.

 

[Jake M]

Close to 20 tpi, BSP has 19 threads/inch, and NPTF (fuel variant of US standard) has 18 threads/inch. Twenty is... an
oddball.

19 and 18 tpi would be 3/8 pipe sizes. All NPT variants an both BSP variants use those threads. That's in the neighborhood of 16mm outside diameter. The OP has measured 9.45mm outside diameter on the mystery threads. His measurement is within a fractional millimeter of 1/8 pipe thread in either system, That's where the 27 and 28 tpi comes from. Or a 3/8 BSF bolt thread, which would be 20 tpi.

 

[RJSakowski]

Queuing on the info in post #1, the thread was measured as 9.47mm (.373") and 20 tpi. It also appeared to be a straight thread. Since it was a pressure sender, it would have to have some means of sealing the connection. A typical method for straight threads would be to use a flange and some type of gasket. Another would be to use a sealant on the threads.

It is unclear what the intent of the OP is. If it is to replace the OEM pressure sender and he is unable to find a stock replacement, then finding a n aftermarket sender with suitable threads would be desireable. Modifying threads on a current sender would be a second choice. Modern senders would likely use a 1/8"MPT. This could be modified to 3/8-20 thread which would retain the original engine threading. Alternatively, the block could be drilled and threaded for 1/8" - 27 FPT.

Looking on a Land Rover forum, it appears that the sender is actually a pressure switch feeding an idiot light on the dash. This type of switch is is fairly simple and straightforward. Any replacement switch with the right pressure setting should be able to interface with the dash light.

If it is feeding a pressure gauge on the dash, then one would have to know what type it is. Older gauges were simply fluid pressure gauge connected to the sender via a small hollow tube to to transmit pressure from the sender. The sender was simply a fitting. A more current type would be an electrical gauge with the sender actually converting oil pressure to an electrical signal. In that case, the sender would have to match the characteristics of the OEM sender which is a more difficult task.

 

[Chewy]

Here are a couple of thread guides that I have. May be able to identify the thread. I am looking for one in particular that has British cars,but can't locate it real fast, If I do I will post it.

https://www.gewinde-normen.de/en/index.html

https://gerardsgarage.com/supportfiles/Threading_Tables.pdf

You can also cut off the body and leave just the threads. Then silver solder an adapter to SAE or 1/8 or 1/4 pipe thread and use a sender that has the same pressure rating you want. I just changed the sender on my Dodge and am going to make an SAE adapter so I can use my test gauges.

 

[Hobby2409]

In response to a couple questions - I need this info to replace my broken sender with a new one. The sender is actually refered to as a transmitter in some of the older literature but everyone these days seems to call it a sender or sensor. It is connected to the oil filter in a Rube Goldberg fashion via some other parts and has a metal plate inside that flexes as the oil presses on it. This changes the resistance of a bimetal resistor inside, which is connected to a gauge on the dash. I think the seal is maintained via some copper washers. The "idiot light"/warning light is elsewhere on the dash and connected to a separate "pressure switch" which (I think) goes on/off at a critical pressure. The switch is missing on my truck and I haven't tried replacing it yet. Since people are curious, here are a few photos:

Screenshot from a youtube video: The sender is the big can in the middle with the 2 terminals. The pressure switch is in the lower right of the main and inset photos (with the single terminal).



Here's what is inside the sender:



And here is the thread I'm trying to ID (attached to the remains of the sender):

 

[Jim F]

In response to a couple questions - I need this info to replace my broken sender with a new one. The sender is actually refered to as a transmitter in some of the older literature but everyone these days seems to call it a sender or sensor. It is connected to the oil filter in a Rube Goldberg fashion via some other parts and has a metal plate inside that flexes as the oil presses on it. This changes the resistance of a bimetal resistor inside, which is connected to a gauge on the dash. I think the seal is maintained via some copper washers. The "idiot light"/warning light is elsewhere on the dash and connected to a separate "pressure switch" which (I think) goes on/off at a critical pressure. The switch is missing on my truck and I haven't tried replacing it yet. Since people are curious, here are a few photos:

Screenshot from a youtube video: The sender is the big can in the middle with the 2 terminals. The pressure switch is in the lower right of the main and inset photos (with the single terminal).

View attachment 460712

Here's what is inside the sender:

View attachment 460717

And here is the thread I'm trying to ID (attached to the remains of the sender):
View attachment 460721

Looks like a NPT in 1/8" flavor.

Just an FYI, a sender controls a gauge, a switch controls a light.

 

[Jake M]

...It is connected to the oil filter in a Rube Goldberg fashion via some other parts and has a metal plate inside that flexes as the oil presses on it. This changes the resistance of a bimetal resistor inside, which is connected to a gauge on the dash...

I stand corrected, I thought it was older, but you do have the goofy one. Cool.

I think the seal is maintained via some copper washers.

That correlates with what you said, that it appears to be straight, not tapered. The photo looks a little bit tapered, but that might just be an artifact of the camera. The threads look to my eye to be too coarse for a pipe thread. You said back in the beginning something about a matching gauge marked 20G. I have not seen a gauge marked that way. Is that 20 threads per inch? If you can confirm yes, then that would surely be a 3/8 BSF thread. British Standard Fine.

 

[Chewy]

There are a couple of Land Rover Forums that talk about the operation of the oil sender. Here is one.
https://www.lrfaq.org/index.html

Question? Are you just making the vehicle drivable or are you restoring it to OEM for a show or auction?
If drivable, you can silver solder an adapter or single point the threads on a piece of brass and use a current model gauge of your choosing.

The fitting that the gauge fits into is a banjo fitting. Real common on brake systems. Uses a copper washer on each side to seal up. It could be threaded on the outside for an adapter or you could make a new one from scratch to fit a common thread.

Haven't found any source for a direct replacement.