No vent in catch can

1/8 is plenty until you have a "big blow" for some reason. "the right" way to do it would be to say, tap a hole in the top of the can and screw in a fitting for an overflow tube. Otherwise, if something happens, it can blow coolant all over the place in there in case of a bad overheat, etc

I'd want say, 1/8" pipe thread hooked to 5/16tube/ hose