Caps for wing tanks need a special positive pressure vent. The top of the wing is, of course, a low pressure area and without a positive pressure vent the tank will not drain in flight. The positive pressure vent is simply a tube that faces into the airstream. (Photo 1)
These caps can be purchased, but they are quite expensive. Make your own for just a few dollars. Start with a tractor gas cap, the kind used on a Bobcat loader is just right, it has a neoprene gasket that is easy to remove without damage. You'll also need 4 inches or so of 1/4 inch copper tube.
Make a cap adapter for your vice. I made mine out of two hardwood blocks. Bandsaw the cap radius into both blocks, and cover the "jaws" with rubber from an old bicycle inner tube. (Photo 2)
Clamp the cap in the vice upside down, drill a small hole (pilot hole) through the center of the spring. Then use a carbide tip drill bit to drill a 1/4 inch hole through the spring, but not all the way through the cap. You'll need a carbide tip drill (cement drill) because the spring is hardened steel and a regular drill bit won't do it.
Flip the cap back right side up and drill a 1/4 inch hole in the top of the cap with an ordinary drill bit. Use the pilot hole for alignment.
Then insert the copper tube and solder it in place. Use silver solder. (Photo 3) The cap has 2 little vent holes on the inside that won't be needed anymore so flip the cap back upside down and solder closed the little vents in there.
Sand or sandblast, paint with white epoxy primer and reinstall the gasket. Install the cap on the tank filler neck, then bend the tube in the airstream direction (forward). I use a little homemade fiberglass bending tool (Photo 4), but a spring bender works good too. Don't try to just bend the tube by hand...it'll kink. Finally paint your fuel cap red.
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Link to Fuel Gauge, Fuel Tank
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