Abstract: The invention relates to a fuel tank cap comprising a body, the upper portion of which is provided with a hole. Said hole exhibits a plurality of concave or convex sections which are formed along the inner periphery of the hole. A cover comprises a sleeve, which is formed on the inner periphery of the cover for insertion into the hole of the body, and has a plurality of axial slits. The sleeve is provided with a plurality of convex or concave sections formed along its outer periphery so as to mate with the concave or convex sections of the hole, respectively. When a clamping torque applied on the cap exceeds a predetermined value, an idle-rotation mechanism is established between the sleeve and the hole. The fuel tank cap and, more particularly, such a type of cap can effectively prevent the leakage of fuel in a vehicle fuel tank when the vehicle collides with another vehicle or object.

Abstract: A cap for a fuel tank has a cap body for screwing into the filler neck of the fuel tank with an "O" ring interposed therebetween, and a lid member for mating to the cap body, both the lid member and the cap body having coacting structure for mutual interengagement to maintain the lid member normally mated to the cap body but separable in break-away relationship from the cap body to leave the cap body intact within the filler neck upon application of external force, such as through collision, to the lid member. The coacting structure may take the form of engaging recesses, holes or grooves on one member and of engaging projections, tabs or pins on the other member.

Abstract: An O-ring for providing hermetic sealing between a fuel tank filler neck and its cap has a single main body with an integrally formed annular lip projecting outwardly from the main body; the lip has an inclined lower surface for contacting a corresponding inclined upper surface of said filler neck, such lower surface defining an angle of inclination .theta..sub.1 with respect to the horizontal principal extent of the O-ring body, the filler neck upper surface defining an angle of inclination .theta..sub.3 with respect to horizontal, .theta..sub.1 being substantially less than .theta..sub.3, the lip being deformable upon tightening of said cap to bring the lip lower surface into hermetically sealing contact with the filler neck upper surface.