I want to drill a tiny hole in my thermostat to help get rid of the air which is trapped behind the thermostat after replacing my coolant. Unfortunately, my car has only one bleeding screw which is before the thermostat. I just want to make the hole for air bleeding purposes (not for bringing the temp down) but I'm worried about engine temp when I'm cruising at highway speeds. My thermostat is rated at 83 degrees Celsius and engine temp on the highway is around 80. What I want to know is whether one tiny hole in thermostat can cause the engine to run cooler or not? I don't want to make my engine run cooler than normal.

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    All cars have a method of bleeding that will rid the cooling system of trapped air. No engine requires drilling holes in order to free trapped air. Thus, if you have air trapped in the cooling system, the real problem is improper or ineffective bleeding. Because engine designs differ, bleeding methods differ. Please identify the make and year of the vehicle, as well as the type of engine; someone here may be able to advise you how to correctly bleed the engine. And...why do you think there's trapped air? Temperature gauges in cars are often very inaccurate. Commented Apr 5, 2021 at 0:11
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    Every thermostat I have bought and fitted, or fitted as part of my work has had a small bleed hole, sometimes with a little ball valve included, to allow the bleeding process to be effective. This sounds like you have not completed the procedure correctly for your vehicle. Check out the correct procedure and follow it.
    – Solar Mike
    Commented Apr 5, 2021 at 5:57
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    In all the cars I have bled, you need to run the engine with the radiator cap open so that the pressure doesn't build up, this allows the engine to get up to temperature and circulate out the last of the air when the thermostat opens (and allow you to top up the coolant as required, you shouldn't need to drill a hole anywhere.
    – Mauro
    Commented Apr 5, 2021 at 9:44
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    If you're worried about 80degrees C being too hot, most engines are designed to run higher than that. And even at 100degrees C, that's not boiling point, as the water is under pressure. See Boyle's Law.
    – Tim
    Commented Apr 5, 2021 at 16:35
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    The thermostat in the picture doesn't have a hole at the edge, but we can't see what, if anything, is going on under the metal "bridge," or in the area just inside the circular frame. It is certainly possible that there's a bleed orifice somewhere. I stand by my previous comment that no car requires the owner to drill through parts in order remove air from the cooling system. Commented Apr 5, 2021 at 21:32

3 Answers 3


Waste of time. Air can already leak through the thermostat.

Thermostats are baffles, not seals. The difference is a seal prevents all flow, a baffle only prevents significant flow. Baffles are used when the same fluid is on both sides and when a small amount of leakage is tolerable.


Yes, you can drill a small hole in your thermostat if you're bothered that you won't be able to bleed the system without it and no, it won't make any appreciable difference to the operation of the system

If you think about it, the effect of turning your car heater on will have a greater cooling effect on the engine than allowing a 1 mm wide low velocity stream of water through the radiator. If a car engine could be effectively cooled by a tiny stream of water then radiator hoses wouldn't need to be 2-3 inches wide

If you remain bothered about drilling a hole perhaps instead consider wedging the thermostat open a tiny amount using a chunk of something that will melt (wax) or dissolve (a fragment of a boiled sweet)* so that it will go away but honestly, as the commenters have indicated, the designers of your car's cooling system knew what they were doing and unless you're certain that your car came with a holed thermostat (that you've since replaced with a non-holed one) and you've certainly caused yourself a problem, then I wouldn't worry about it

* I wouldn't personally do these things because I don't think there is a need, but i offer them as ideas because it may help you sleep at night

  • Thanks for your comment. My thermostat is OEM and it comes with no holes in it. My problem with the car is that it only has a single bleeding screw while most cars have two screws (one before and the other after the thermostat). Therefore, I can't bleed the system when the engine is cold and thermostat is closed. I fill the system with coolant but there is still some air trapped in the system. To complete the bleeding process, I put a funnel on radiator neck and start the engine and let it warm up to open the thermostat and the remaining air will exit the system but long idling is harmful.
    – LFY MP7.3
    Commented Apr 5, 2021 at 6:57
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    Heat the thermostat (after you fitted it) with a hot air gun so it opens, then, or boil it in water and fit it while hot then refill before it cools, if you have time. Or wedge it open slightly as I put in my answer. Or drill a hole in it as I put in my answer. You said most cars have two screws - I don't think I've ever owned a car like this, and I've owned a lot of cars. I also don't think I've ever put so much thought into bleeding a cooling system - I think the biggest problem here is that youre overthinking everything. Mechanics don't think this much
    – Caius Jard
    Commented Apr 5, 2021 at 7:04
  • Yes, that was an answer. Sorry for the mistake! I also thought about heating the thermostat before installing it but it takes about 10 minutes to install my thermostat cover because one of the bolts can only be fastened by a wrench which takes time and the thermostat will close again. My idea is to find a tiny hose and use it to wedge thermostat open. Then I can pass the other end of the hose through the bleeding hole. This way, After filling the system I can pull the hose out of the system through the bleeding hole and thermostat will close without any trapped air. What do you think?
    – LFY MP7.3
    Commented Apr 5, 2021 at 7:15
  • I think you should drill a 1mm hole in it
    – Caius Jard
    Commented Apr 5, 2021 at 7:32

"I think you should drill a 1mm hole in it"

It is clear that you already made up your mind to drill the hole no matter what anyone advised. The fact that manufacturers do not drill a hole is enough. If it was necessary then issues would appear straight off the showroom floor.

There are jiggle pins through a hole or some kind of compensation for burping a system therefore moot to attempt any modifications that aim to achieve that which has already been solved.

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