Well, I know what WON'T work! The cooling fan switches for modern cars turn on at 220 degrees, way too hot to be of any benefit for the average stovebolt engine. Try a safety shutdown switch for a tractor-trailer engine like a Cummins diesel. I think some of them are adjustable, or are set up to turn on a fan, or kill the fuel control, at a lower temperature. 180 degrees should be about right for your needs, I think. Try your local big-truck dealer's parts department, like Freightliner, Kenworth, or Peterbilt.

Edit: I just re-read your post. If you're running a modern engine, and a radiator capable of carrying 15 lbs. or more of pressure, just use the fan switch for the car the fan came from. It will turn on at 220, and turn off again at about 190 or 200.
Jerry

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