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 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 and no, we do not charge you a credit card fee.
*The 400 and 351M had this feature cast into the block and did not utilize this brass bypass. If you have the 400 or the 351M you can purchase the 330 series. The cast bypass will be partially sealed off.