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Add electric fan temp sending unit in this hole, odd thread size?

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I am adding electric fans and aluminum radiator to my 73 240Z. I would like to put a coolant temp sending unit (on @ 185, off @ 165). The sending unit that came with the kit is 5/8” NPT -no good.

My thermostat housing has an unused port (pic below) that was plugged with an old sensor of some type ( on the right, pic below). I can’t ID the thread size on the fitting. The temp sender (on the left next to it) is 1/4 NPT.  The threads on my mystery port measure slightly less than that - more than 18, less than 20. 

I get the feeling that this is not the original thermostat housing for my car, because I can’t find a mention of this second sensor or this hole.


1. can someone ID this housing, and tell me what the sensor that is in this hole did?

2. Can someone ID the hole size and thread pitch?

3. ultimately, I need to find a sending unit to fit in this hole that will operate a fan relay. Anyone been here before me, had success in finding a sender?

long post; TIA!

Edited by Trimpe.bob@gmail.com
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