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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!



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You are correct about it not looking stock. It looks more like a 260Z housing. (I just went down to the garage to look.) For reference my 73 does not have that fitting, but my 74 does.

@Captain Obviousmay have the size memorized. It could be a BSP fitting. Anyway, the fitting is for the water temperature switch for the 260Z EGR control. If you are really curious, see page EC-16 in the 74 FSM.

The switch is still on my 74, doing nothing. When I needed to install a coolant temp sensor for the fuel injection I put on that car, I used the fitting on the side marked below.


It's 1/2inch BSP. I found the right size BSP to NPT adapter for the coolant temp sensor. The 74 already had the coolant hoses removed from the carburetors.

I plan to remove the coolant hoses from my 73 soon. I just have a BSPP plug for the side fitting. I bought an elbow fitting from Napa to replace the three-way coolant pipe on the passenger side of the block.

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Thank you, SteveJ. I found the description in the ‘74 manual. It is an on/off switch.  I thought about just replacing it with a good one and using it,  but the temp range is too low for what I wanted to use it for.

I read a couple of statements that 1/4 BPST and 1/4 NPT will play together, but my 1/4 NPT adapter only screws in the hole less than one full turn. I don’t think I’m going to force that one! I found a chart below that spells out the differences.

I’m going to try to find a BPST to NPT adapter. Barring that, i think I’ll look at drilling and tapping this hole to accept the 3/8 NPT sensor that I have. It may be less work than un-plumbing and plugging the carb heater line.

edit::: I’ve reconsidered. I don’t think there’s enough metal around that hole to allow a drill and tap. And, an adapter won’t work, because the sensor has an un-threaded end and won’t fit into an adapter. See pic. I think I’m back to using hole #3 like you suggested.

Be glad to hear more points of view. I’ll post up when I have some news. Thanks for the help!



Edited by Trimpe.bob@gmail.com
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I went ahead and removed the carburetor heater plumbing. Wasn’t so bad. I used the hole to install the sending unit. I used a 1/2” NPT adapter in this BSPT hole, added some Teflon tape to prevent spiral leaks. 
Don’t have it wired up yet but the water circuit is leak-free!

Edited by Trimpe.bob@gmail.com
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