I finally rebuilt the snowblower gas cap, this time before the splash cone worried its way through the plastic stem and dumped all the parts into the tank.
The black rubbery domed thing conceals a chunk of open-cell foam that evidently separates the gasoline from the vent channels under the rim. The plastic stem was quite eroded, but probably had another season or two left in it; now I need not worry about it.
Unlike with the shredder cap, this time I put the screw head on the outside and the jam nuts on the inside; there’s also a jam nut under the cap to keep the screw from walking outward. Just in case, I wrecked the threads to keep the nuts from walking off the end, too; you can see the gouge just above the nut.
The Smell of Molten Projects in the Morning 