Series 3 temperature gauge help required

I have a 1975 series 3 petrol which was restored by the previous owner and the Jaeger temperature gauge does not work. The wiring loom was replaced with a professionally built one
The engine is a 2:5. So I thought the simplest thing to do first was exchange the sensor before pulling our the dash. However I’ve now bought two, ones the wrong size and the other is a to courser thread, so no further on.
3 Questions
Is the sensor that is already fitted likely to be compatible to jaeger gauge or would it require a different type.
Can you buy a sensor that is compatible to the jaeger gauge that has the correct thread.
The one fitted looks to be a fine thread and is 15mm wide.
How would you go about testing the sensor and the gauge.
First time post thanking you in anticipation
R Morris

You can see if gauge works by with ignition on ground the wire that is at sender on the block, if good gauge will go all the way up. Does your fuel gage work? On the same fuse.

tottot said:

You can see if gauge works by with ignition on ground the wire that is at sender on the block, if good gauge will go all the way up. Does your fuel gage work? On the same fuse.

Click to expand...

Thank for the prompt reply
The fuel gauge works fine and I have just earthed the sensor wire and the temperature gauge goes up. The sensor that I have bought appears to be a unc thread and the one fitted looks to be fine thread. Presumably providing I replace with a like for like part number it should work, do you think?
Thanks again

Ok gauge good. You say your engine is a 2.5 unit? That so it will have come from a Ninety or One Ten model and need sender for that engine.

Ok thank you, the one I bought was a Lucas, the thread are quite different
I will have a ring round on Monday, quoting the part numbers
Thanks
R Morris

Worth testing sensor in heated water while connected to temp gauge to check it covers the range reqd.
Sensors need matching to the gauge to get accurate readings as they have differing Temp v resistance graphs

Thanks for the advice, in the end I needed to buy a 2:25 temperature sender which matched up to the jaeger gauge. The 2:5 sensor is a different size and was not compatible with the jaeger.

Famous Four managed to source an adapter to allow to fit into the block .
Thank you