Abstract: A plastic cap for insertion into the filler spout of a fuel tank including a base member with a downwardly extending sleeve and one or more annular outwardly extending ridges adapted to pass through and subsequently engage the interior neck of the filler spout securing the cap in place. The cap further includes a cover member integrally attached to the base member and adapted to seal the filler spout when placed in the closed position. When the cover member is in the open position, access to the filler spout is provided.