Robertshaw 333 Series hi flow balanced sleeve thermostat

Robertshaw 333 Series hi flow balanced sleeve thermostatThe 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

The 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 performance.   It was an ideal solution for overheating 351C found in mustangs and hot Panteras. 

The OE Robertshaw thermostats for the OE bypass is available again.  Its made in the USA.  Yes this is a real Robertshaw 333 series.  If you want 160 degree alternative, consider the 333-substitute

The substitute performs the same preheat function but we resized the bypass hole for more temperature options.  

*The 400 and 351M had this feature cast into the block and did not utilize this brass bypass insert.  The cast bypass hole requires the 333 series.