Robertshaw 333 Series hi flow balanced sleeve thermostat with bypass plateThe Robertshaw 333 series thermostat has the distinction of being the only available thermostat that would accommodate the 351 Cleveland Controlled bypass system*. A 351C provided greater water flow in the engine for a faster, more even warm up without creating hot spots (particularly in the valve area). The bypass design controlled the water flow and temperature with less cycling and increased the flow to the radiator for improved cooling