some people put the hole in it to facilitate easier bleeding especially the first time its driven but i dont mess with it, i just let it idle for a few minutes with the front of the car higher with the cap off and then take the car for a ride(cap on) and the temp can shoot up a bit but as soon as the thermostat opens you never know the difference. if the car has bleeders they need to be open when filling the car with coolant, this is done fairly slow and that keeps most of the air out in the first place