The part is not ret received so the following is theory.  The apparent reason for breakage is due to slight shifting during installation of the new thermostat from its intended very shallow counter bore onto the flat surface reserved for the mating housing.  This causes the thermostat outer radius to create a high spot (should be flat when nestled properly) onto which the housing is torqued, snapping the casting. I intend to lock the thermostat into its C-bore with a thin coating of liquid gasket before mating and torque of the housing.