is to form material from the outer sheets 126 128 in fig7 a a first extrusion 148 formed by interaction of a male punch and a female die not shown extends from the periphery of the opening in the first sheet 126 a sufficient distance so that the flange of the form lock 144 will not overlap with the inner sheets 130 in fig7 b a second extrusion 150 extends from the periphery of the opening in the second outer sheet 128 beyond the first extrusion 148 and through the aperture 134 as shown in fig7 c the second extrusion 150 is then folded back over the first extrusion 148 the only sheet within the flange of the second extrusion 150 is the extrusion 148 of the first outer sheet 126 the thickness of the form lock 144 is therefore equal only to the thickness of sheet 126 added to twice the thickness of sheet 128 therefore in this embodiment as long as the inner sheets 130 are at least as thick as the outer sheet 128 the resulting form lock 144 is no thicker than the rest of the gasket 120 and can be confined within the planes 136 138 defined by the outer surfaces of the gasket because form.