the rest of the body is disposed therebelow also for convenience of reference a cartesian coordinate system comprising a vertical axis z z disposed at the center of the grommet as best understood with reference to fig8 may be visualized in such a reference system of cartesian coordinates the other two axes x x and y y being oriented as best understood with reference to fig5 the plane defined by the intersection of axes z z and y y will be referred to as the principal plane of grommet 200 various dimensional relationship will then be understood more readily at a predetermined distance 34 z 1 34 below the lowest point on flange 202 in its unstressed state on the outer surfaces of both principal sides of grommet 200 are provided straight horizontal shoulders 208 208 each of a length 34 1 34 along the y y direction as best understood with reference to fig4 the outer surfaces of grommet 200 immediately above shoulders 208 208 in their unstressed state are separated by a distance 34 a 34 and the shoulders themselves extend outwardly so that the distance between their outermost edges is 34 b 34 as best understood with reference to fig5 the outer surfaces of the principal walls of grommet 200 i e the walls substantially along the principal plane then taper downward to blend into substantially parallel portions separated from each other by a distance approximately.