Fastening element of synthetic material

A one piece plastic fastening element, adapted to be received in a complimentary hole of a predetermined diameter in a plate-like support, including a cup-shaped hollow insert member open at one end, said insert member having a continuous wall and an external substantially continuous circumferential shoulder formed integrally intermediate the ends of said insert member for engaging beneath the underside of the plate adjacent the edge of the hole, a head member connected in spaced relation to the insert member having means for fastening an article to the carrier plate with either the article to be fastened or the head member lying in close contact against the side of the plate-like support opposite the side engaged by said circumferential shoulder, and connection means extending between, and substantially parallel to the axis of, the insert member and the head member, said connection means extends along the inside of the insert member axially below the circumferential shoulder, said connection means includes at least two connecting webs which are circumferentially spaced and connected at the upper ends thereof to the head member and at the lower ends thereof to the interior of the insert member below a plane passing transversely to the axis of the fastening element and through said circumferential shoulder, said insert member further includes a lower cup-shaped portion having an upwardly and radially flaring wall extending from a closed end, a generally cylindrical portion extending axially from said upwardly flaring wall, and a portion extending axially from said generally cylindrical portion having a reduced external diameter forming said continuous circumferential shoulder, whereby said reduced diameter portion forms an axially extending thin walled annular extension with a substantially unrestricted open end, said external diameter of said reduced diameter portion being slightly greater than the diameter of said complimentary hole whereby said extension is adapted to agressively seal against the side wall of said hole, said head having an opening therein axially aligned with the interior of said cup-shaped hollow insert member thereby permitting access to said interior through said head.