By putting a .75” (.775 or 20mm) core plug in the hole in the front of the block passage that leads to the thermostat, it will restrict flow. my theory of reason is that i want most/all of the hot coolant to go back to the radiator and not recirculate so my fan sensor will cycle. it was running constant once it came on the first time, it didn’t get cool enough to shut off. i want to give the plug credit for dropping the temp at my fan sensor enough to allow the fan to cycle, but outside temps were much lower when i got it running but the fan did cycle.
Slower warm up time:
The heater hoses make a bypass circuit also; it’ll keep the system from super-pressurizing before the stat opens too.
The Weiand aluminum water pump actually blocks that hole. I don’t remember if the Milodon pump does also or not, but I know the Weiand pump does. -tinman351
Better vent the t-stat
I don’t think you want to completely block off the coolant bypass unless you drill several holes in the thermostat. If you don’t there will be no coolant circulating within the block at all until the t-stat opens. That seems like it would cause a problem with uneven heating. I have blocked mine off in the dirt tracker, but I don’t run any thermostat at all. –rgsrace