Abstract: A thermostat assembly for an engine cooling system having a coolant flow passage extending from an inlet connected to the engine and an outlet connected to the engine radiator, a thermostat valve positioned in the inlet to control the flow of fluid, a bleed valve mounted in an air bypass passage to allow air past the thermostat valve during filling, and an integral fill passage through which the whole cooling system may be filled; the fill inlet being the highest point in the housing, the bleed valve being positioned at a level higher than the thermostat valve so that air is completely purged from the system during filling; a further separate air vent passage communicating with the bypass passage to vent air from the top of the engine to which the housing is fixed.