I am going to be upgrading to an E fan and and im looking to see what the best temp sensor option is. I was thinking of using one of the plugged holes in the intake manifold by the thermostat housing but I dont have access to the truck. If anyone could tell me what the thread is I would appreciate it so that I can get my parts on order. If thats not going to work I will likely just go with the radiator probe type of sensor.

 

I would guess 1/4" NPT

 

Get a Ron Francis Wiring Fan Control unit. I have them on my 1983 Toyota truck, my street rods, and customs. They are reliable and accurate. I believe they are 3/8" or 1/2" NPT. If you have a water outlet with the boss cast in it just have the boss drilled out and tapped. L C Engineering has the water outlets and they can drill and tap the boss for you. Tip: I drill out my thermostat by pass hole to 1/4" thus reducing temperature over shoot when engine is warming up.