Hole in the Thermostat

[v8s4me]

There's a debate going on on another forum about the little hole in some thermostats. What is it there for? I can't remember

[phillpot]

I've always understood it to be for venting air, particularly when filling the system?




If the thermostat is fitted horizontally it should be at "12 o'clock"

[phillpot]

Looks like others agree ........................

[Classic Chim]

What’s the general view on extra drilled holes to allow a small amount of coolant to flow around the system even before upto temp.
Is it a myth that this also helps bleed air out of the system.

[GreenV8S]

I think a single 1/8 hole to sort out the bleeding is all that's needed.

There is a theory that additional holes reduces the thermal shock when the stat eventually opens. If the stat seals perfectly and the circuit is designed properly so the stat controls all flow through the rad, the whole engine gets good and hot and then has a radiator full of cold water dumped through it when the stat finally cracks open. Having a controlled leak can prevent the radiator from being completely stagnant and soften that shock. Too much of a leak and the engine will take too long to warm up, and over-cool in cold weather. Too little, and it won't make much difference to the shock.