The ACS twist-to-lock nonvented flush fuel cap is machined from 6061 aluminum billet. The sealing element for the cap is an easily replaceable viton “O” ring. The bottom of the cap is secured to the bottom of the ring by a safety chain to prevent loss of the cap during flight. the mounting ring may be welded, riveted, bolted or molded onto the tank. The bottom side of the mounting ring has 6 center drilled marks (they do not go through) to aid in drilling an evenly spaced 6 hole pattern if required.