The gas cap is not a sealed system to begin with... It's vented.
The real propose of the cap is to keep contamination like dirt and grass from getting into the tank:smile:.
So ... I guess I am with him it won't matter that much if it is doing it's primary job.
HOWEVER if it is a craftsman mower you might start looking now for the correct size and thread count cap...
I couldn't believe that everyone else uses one of three or four different gas caps... but not craftsman
theirs is completely different (but looks similar) and I have no idea how many variants of differant they have. KennyV