have you had one on a car for a very long time yet? My concern is how long will you be able to see the "stat" before the coolant stains up the housing 'til you can no longer view it workings.
The first prototypes I made were a year ago. All 3 are still on the trucks I installed them on and are driven on a regular basis. You can still see everything fine with no buildup on the inside of the glass. The viewable tube is made of steam boiler site glass. (Pyrex) The surface will not cloud up like plastic does. Also part of the problem with the see through valve covers is the oil builds a coking layer (burned oil) on the plastic. This doesn't happen with coolant.
