Expandable nuts or so-called grommets of a plastic material have become known in most varied designs. Commonly, they are snappingly inserted into the opening of a sheet-like support by means of a shank. The shank has snapping means to retain the grommet in the opening. Furthermore, the grommet has a flange-like head which sealingly comes to bear against the side of the support that faces it when the shank has been snappingly seated in the opening. The head frequently has a flange-shaped portion of another plastic material which is softer than that of the shank and has a better sealing effect. The shank has an inner axial receiving bore into which a member having a male thread can be screwed to fix another member to the support.
The receiving holes in the support member frequently are of a square shape. Accordingly, the shank is also of a square shape in cross-section. In this context, it has also become known to provide shoulders at the outside of the shank at least on diagonally opposed sides close to the head. The shank and the receiving opening are profiled and dimensioned so as to allow to rotate the shank about a certain angle. As a result, the surface of the shoulders gets under the support member on the side opposing the head so that it is no longer possible to draw the shank out of the opening. At this stage, the surface portions above the shoulders cooperate with the edge of the opening to clampingly retain the grommet in the rotated position in the receiving opening.
Naturally, the sheet of the support member is with tolerances. In addition, the support members are of different thicknesses which depend on their application. The distance between the shoulder and the underside of the head cannot be prevented from being larger than the thickness of the support member. This can cause the grommet, when under a load, to come loose from the support member. Besides, this endangers the water tightness, safe function, and freedom from rattling, which are required particularly for the manufacture of automobiles.