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. Unfortunately, due to slowing sales, Robertshaw ended production of the 333 product.
When we discovered the hat modification and plate was sold for $125.65 without the thermostat we thought there has to be a more economical way. We designed an alternative that will utilize the Robertshaw 330 series hi flow thermostat in the existing 351C controlled bypass system. Simply remove your bypass and tap the new bypass into the counter-bore until it seats. That’s all there is to it. Most ran these engines with a 192 degree t-stat but the 330 is series available in 160, 180 and 195.
Insert and the Robertshaw thermostats are both Made in the USA.
*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.