When you receive a new aftermarket gas cap, do not assume that it is vented. I haven't bought one over the past 15 years from our normal sources that were indeed vented as the originals. They all have plastic parts in them which does something I'm sure, but venting is not one of those things. When I get one of those, I carefully drill a 1/8th inch hole in the plastic disk viewable in the underside of the cap.
No need to drill past that disk. That is where "carefully drill" comes into play.
