A bulkhead retaining nut for AN6 fittings, threaded 9/16 inch-18 to match the bulkhead shank. Wound on from the reverse side of the panel it clamps the fitting firmly in position, which is the whole point of a bulkhead: the panel takes the mechanical load and vibration instead of the hose ends, and the line passes through cleanly with a proper sealed joint rather than a rubber grommet. AN6 bulkheads are extremely common on firewall pass-throughs for fuel feed and return lines and on surge tank plumbing. Check the shank length suits your panel thickness, and avoid over-tightening into thin sheet metal without a reinforcing plate behind it.