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.
Questions?  Comments?  Send email to: