Lucerne. A circular disk covered in rubber or plastic with a hole in The center where The edge is sometimes tensioned by a spring that prevents fluid from leaking at The Engine Coolant Outlet Housing. Engine Coolant Thermostat Gasket. Engine Coolant Thermostat Housing Seal. Engine Coolant Thermostat O-Ring. Engine Coolant Thermostat Seal. O-Ring. Thermostat Housing Gasket. Cooling. Housing. O - Ring. outlt. Water Neck. Water Outlet. Included with: Thermostat.