Thermostat bleed hole

[lloyd]

I recently pulled the thermostat from an early 530 (leaking O ring) and noticed the thermostat had a 2mm hole drilled in it. Years ago this was considered a routine modification that "possibly" added a "slight" safety margin against overheating. (It probably slowed function of the thermostat by bypasing it to a limited degree - which may have been less important to emission control in the 60's and 70's.)

I looked through the old thermostat collection and noticed that most of them had similar modifications.

Does anyone still do this with their thermostats? Why?

Thanks in advance.

 

[Philip Slate]

The theory was that the thermostat be positioned so that the hole was at the top when installed to allow more complete bleeding of air from the system, hence less likelyhood of overheating.

 

[Malc]

Philip is correct it's to ensure that all or as much air is out of the system as possible.
Good aftermarket ones still have them, often with a "jiggle" pin in them.
My Saab 900 16v hack being an example
Any thermostat that is mounted away from vertical should have one.
It also allows a little warming water to flow past any coolant temperature sensors for modern FI systems etc

Drillinga 2mm hole in one to prevent over heating isnt going to work. If the thermostat is jammed shut the car will over heat. Conversely never run without one as this will effect the coolant flow through the engine and lead to localised "hot spots", usually cylinders 5 and 6, a long way from the water pump.
You can use a flow restrictor plate which has a flow area the same size as an open thermostat. This is a trick often used by the racing crowd.

Malc