Ponton, I found out the new tanks don't have the same depth at the neck to hold the cap so the old caps don't fit correctly (the rollover area inside the neck). What I did was I have some rubber pads that are resistant to fuel about 1/16" thick from my previous employer (molding business). I cut out two rings to fit in the cap, and now viola, the old "not vented" cap works like a charm.