That's a bellows-type thermostat. As the coolant heats up air inside the bellows expands and opens the disc valve. They work best in cooling systems with very low or no-pressure radiator caps. I'd consider a 7 lb. cap to be about maximum for that type of stat and a 4# would be better. Higher pressures tend to make them stick open or closed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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