How reliable are AutoZone thermostats for a Mazda RX-7 FD?

I strongly recommend avoiding AutoZone or other aftermarket thermostats for your FD RX-7 and sticking strictly to the OEM Mazda thermostat ¹.

Aftermarket units are often unreliable when it comes to their true operating temperatures. For example, some aftermarket units (like Stant) rated for 180°F have been tested and shown to not open until around 205°F ¹. By contrast, the OEM Mazda thermostat is highly reliable and engineered to begin opening precisely at 180°F (82°C) and reach fully open by 203°F (95°C) ¹.

Testing a Thermostat If you suspect your current thermostat is stuck shut, you can safely remove it and run the car temporarily to see if normal coolant flow resumes, or you can drop the thermostat into a pot of water on the stove with a thermometer to observe exactly what temperature it opens at ¹. Do not leave the thermostat out permanently, however, as running an FD without one actually leads to worse overall cooling performance ¹.

Task Difficulty: Easy Replacing the thermostat on the FD is a straightforward job. You do not need to remove the water pump housing. The thermostat sits right behind the filler neck, which is secured by just three 10mm bolts ¹. When pulling the old one out, just be sure to make a note of which side faces outward so you can drop the new OEM unit in correctly.