A co-worker was cleaning out his garage (divorce) and gave me this.

Is it a proper gage for tranny fluid temp? If so, how should I go about mounting it in the fluid - Pan? Cooler supply line? Cooler return line?

 

Judging by the temperature range on the gauge, it sure looks like a genuine mechanical transmission gauge/sending unit to me. Generally you want to place the sensor in the fluid thats in your pan, since it is the temperature that your clutches/seals/bushings/etc. has just seen, which is the most important and relevant temp. The line going to the cooler is going to show higher temps (since it is right out of the converter), and coming out of the cooler, naturally will show the coolest temps. You'll want to weld a female NPT fitting in the side of your pan to install the sensor bulb.

 

I hadn't looked at it very carefully, assuming it was a mechanical coolant temp gage. I was out at the swap meet today and a guy said, "That's an awfully high range on that gage." I looked at it again and realized it called it self "oil temp".

I guess I'll have to find an NPT fitting that I can braze (prefer that to welding) into the pan.