Abstract: The opposite ends of a flat strip of fluorocarbon resin tape of predetermined length and uniform width and thickness are butt-welded together to form a continuous circle of flat tape having uniform width and thickness throughout the length thereof. The opposite side and edge portions of the continuous circle of flat tape are passed between rolls which press and circumferentially stretch or elongate said portions to increase the peripheral circumference of the outer edge portions of said tape substantially to the outer diameter of an annular inner gasket assembly with which the cover is subsequently combined. The pressed and elongated circle of tape is doubled about the inner edge of an annular inner gasket assembly to dispose the circmferentially elongated opposite side and edge portions of said tape upon the opposite faces of the inner gasket. The overyling portions of the tape are ironed falt against the faces of the inner gasket by the application of heat and pressure.