A fastener to affix a component in a support fitted with an aperture, comprising an affixation casing of resilient plastic, further a hollow conical shell, and at one end a radial flange that cooperates in sealing manner with the support surface facing it when the affixation casing has been mounted in said aperture, the casing being fitted peripherally with radial bosses, ribs or the like that engage from below the aperture rim when the affixation casing, being temporarily deformed radially at its bosses, ribs or the like is being inserted into said aperture, further comprising a plastic core which is fitted with a head and which can be inserted from the affixation casing's flanged end into the core cavity and which radially expands the casing and prevents it from being removable from the aperture, characterized by at least one radially inwardly pointing boss on the wall of the casing cavity and by the core being fitted at its end with a preferably peripheral groove that is entered by said boss when the core is pressed into the conical shell of the affixation casing and by the core being fitted on the side of the clearance facing the of the core with a shoulder or a shoulder engaging under the boss when the core has been fully inserted into the conical shell.