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Here is a link for a high quality brand
http://www.ebay.com/sch/i.html?_trks...at=0&_from=R40
They use a 22 mm thread diameter. I welded the bung (mid RH tank) It could also be put into a short length of aluminum tubing and inserted with clamps into the lower hose run. The one I use closes (fan on) at around 195 and opens (fan off) at about 182, these are estimates. If you choose one of these you may need to do some temprature chart conversions as they are generally marked in Celcius values. Use this or any other Thermo switch to close the circut for a relay that is hot wired to the fan. Do not allow it to carry the current for the fan directly.