The stock OEM Mazda thermostat is the best thermostat for the FC3S.  On an SPI electric water temp gauge, the thermostat pops open at 190°-195°.  Trying to run a cooler, aftermarket thermostat usually ends up overheating; the aftermarket thermostat tends to open slower than a stock OEM thermostat.

There are aftermarket thermostats available from Japan rotary tuners in Japan that seem to work, but these are not readily available in the U.S. for most people.

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