Bypass style thermostat housings are required whenever a thermostat is being used in the cooling system to provide water to the engines exhaust system during the time that the thermostat is warming up. This is the unit used in crossover cooling system #620-300100. It features (3) 1" push-on discharge connections. A key factor not found in most other thermostat housings. Many thermostat housing do "NOT" provide adequate water to the exhaust system during warm up due to too small of a bypass port.