Abstract: Provision is made for installation, in the interior of a plastic coupling, of a cylindrical tubular-shaped metal insert with internal threads and having an externally knurled external flange at one end, followed by an intermediate section whereon external teethlike serrations or the like are provided, finally followed by a circumferential recess to house an elastic sealing ring, which insert is installed in the plastic coupling after the latter has been molded, and by embedding through transmission of pressure and heat.