[gary678*]

I have a 5.5 head and a 160 degree thermostat with a 50/50 blend of antifreeze. On a hot summer day up a long steep hill on NY's Bear Mountain (2.0 miles+) at 45 MPH the temp went to 170; on the downhill side it dropped to 150. Otherwise it runs at a steady 160. My guess is that if you develop a heating problem that the cause is probably not the head but something such as a blown head gasket with the loss of coolant, or simply a deficient radiator or water pump, or more. Gary