Originally posted by glenmore
The brass bridge on the thermostat was broken in two.
In my portfolio of failure analysis of automotive parts, I had the opportunity to diagose a thermostat with a failed bridge. This phenomenom was traced to "hammering" in the cooling system. The combination of hot coolant, vacuum produced by the suction side of the water pump, and nucleant boiling produced a hammering effect, effectively destroying the thermostat. This same effect can also destroy the water passages in the aluminum head.
Glenmore, was the bridge thinned somewhat before breaking? Make sure the rest of the cooling system is in top condition, that you're running the proper coolant strength, and that the radiator cap is holding the proper pressure.