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.
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