Thermostatic fan switch for sbc.

And the answer is---NAPA part number FS120. This is a brass fitting with 1/2"NPT male end on it that screws into a small block chev intake manifold just to the left of the water neck. When your engine temperature gets to 190 degrees Fahrenheit, this switch will pass current between the two attached wire terminals. The wire from one terminal (it doesn't matter which) goes to a ground, the other runs to the ground side of a 30/40 Amp relay, which will turn on your electric fan. The part costs $29 in Canada, and the relay costs $5, and if you get fancy and buy a premade pigtail that plugs into the relay, it costs $5. The switch I purchased was carried in stock, but the NAPA man wasn't able to tell me what car it was off of.