No, I don't think they are air tight. I think they are actually vented. Fluid that is discharged here is going to boil. It's not part of the pressurized cooling system, just an overflow.
I mentioned the seal to my friend , plus I stated the Buick tank was located higher than generic unit. He said a bucket would basically do the same job. As plastic that is 40 years old heat has distorted both the lids and tank.I was able to get one lid to snap on but I soaked both in our hot tank 200° for an hour to make pliable. Next is to try to use "gasket" on a Electra tank scored from south bend nats years ago. The tanks as seen above were twisted and the o-ring was only thing could be saved on second tank. Now I have enough to make a new ring with material sourced from grainger. But what I don't know is if tank is supposed to be airtight
It cannot be airtight because it has an opening for overflow with hose. Besides that it would blow the top off from pressure....
They are "vented" via the longer, upper overflow hose that affixes to the clip found on the passenger's side tank of the radiator.
