The advantages are that they are lockable, easy to open, don't leak water (they claim).

Here is a picture of the kit, with the two standard Van's caps at the top.

The sleeve goes into the normal gas cap opening, attached via proseal. Note that it does reduce the size of the opening, but we tested the sleeve on a variety of pump sizes, and there is no problem with fit. The sleeve does slightly reduce the view into the cap, but most pumps seem to be auto-stopping these days, so I doubt that will be a problem.

The sleeve slipped into the hole. Perfect fit.

The cap on. Very clean, locks, and does not leak water. I did a tank pressure test, and they don't leak air either.

Update: 5 September 2004

These fuel caps can be bought from Aircraft Spruce as part number 05-01006 and from Van's Retrofit Lockable Fuel Caps