Abstract: A filler cap assembly includes a sleeve welded to a tank wall, and a non-indexing filler cap. The cap is formed with top and bottom members, each member having an outwardly bevelled surface opposing a corresponding surface on the other member. An elastic O-ring is positioned between the two surfaces. The top member and base member are held together by an axially extending shaft, with a spring biasing apart the two members. The shaft is connected in a hinged manner to a cam lever. Activation of the cam lever brings the two opposing surfaces together, forcing the O-ring outward to form a seal with the sleeve. The filler cap and the sleeve's inner wall are provided with additional features which maintain the filler cap in the sleeve.