Installing a new 160-degree Thermostat
(1990 Miata in pictures shown - by Geoff Kim)
(Click on the thumbnails on the left to get the full size pictures.)

Things you'll need:
  • Thermostat (I used a 160 degree unit from NAPA -- NAPA #42, 52mm thermostat).
  • Thermostat Gasket (NAPA#C20115)
  • Blue RTV Silicon Gasket Sealer
  • 12mm Box/Socket Wrench
Follow the upper radiator hose and locate the thermostat housing.
Disconnect the temperature sensor lead from the top of the housing and remove the two 12mm bolts holding the housing in place.
Gently lift off the thermostat housing, you may need to tap it from the bottom to loosen.
Here's the view of the old thermostat and the old black gasket holding it in place.
Using a dull razor blade, carefully scrape off the old gasket.  You do not want to score the aluminum or you will have leak!
Here's the old thermostat removed from the housing...
Here's the new thermostat in place.  It still needs to have the new gasket put in place.
You will also need to scrape off any remnants of the old gasket from the upper half of the housing as well.
Apply a very thin layer of the blue RTV silicon to either the base of the top housing or to the top of the new gasket.
Fasten the top of the housing back on and tighten snugly.  Make sure to alternately tighten each of the bolts to ensure you are applying an even amount of pressure on the housing (ie. do not crank down on one side and then do the other.)  Recommended torque is 14-19 ft/lbs.  But I just hand tightened them in place.  If you notice any leakage, you can always tighten them more later.

That's pretty much all there is to it.  Start your car and wait for it to warm up.  Rev the engine a few times while it is warm and check for any signs of leakage.  Refill your coolant as necessary.