invention at the front side of the mounting guard ring 1 is provided a lid 6 that forms a single unit with and made of the same material as the protective ring 2 the lid 6 is limited by a circular second predetermined breaking site 14 and by an integral hinge 9 by pulling on a grip 8 that is fastened to the lid 6 the second predetermined breaking site 14 is broken and the lid 6 is swiveled away from the protective ring 2 by the integral hinge 9 the grip 8 serves at the same time as a holding device for removing the mounting guard ring 1 after the sealing ring 19 has been mounted fig6 shows on one front end of the protective ring 2 a mounting guard ring 1 with a lid 6 that is made of the same material and as a single unit on the other front end of the protective ring 2 is a holding device 4 that is made of the same material and as a single unit before the sealing ring 19 is mounted the holding device 4 which with the aid of the integral hinge 9 is capable of swinging upward is stopped in the mounting guard ring 1 by a snap connection 16 at the transition from the protective ring 2 to the lid 6 is disposed a circular second predetermined breaking site 14 by pulling on a grip 17 the lid 6 is removed completely from the.