14 CFR 23.973 - Fuel tank filler connection.
(a) Each fuel tank filler connection must be marked as prescribed in § 23.1557(c).
(b) Spilled fuel must be prevented from entering the fuel tank compartment or any part of the airplane other than the tank itself.
(c) Each filler cap must provide a fuel-tight seal for the main filler opening. However, there may be small openings in the fuel tank cap for venting purposes or for the purpose of allowing passage of a fuel gauge through the cap provided such openings comply with the requirements of § 23.975(a).
(d) Each fuel filling point, except pressure fueling connection points, must have a provision for electrically bonding the airplane to ground fueling equipment.