Assembly which can be supplied to a liquid medium, with: a) a plastic cap with: - a basic body, which has a receptacle, suitable for a screw, with an opening, wherein the basic body extends along a longitudinal axis (Xa) between a first and a second portion, wherein the basic body has an outer surface and an inner surface, the inner surface points in the direction of the receptacle, and - a flange, disposed concentrically to the longitudinal axis, with a first sealing ring disposed on the flange circumference, wherein the flange is provided on the circumference of the opening against the first portion, wherein the flange has a first and a second surface, wherein the first surface points in the direction of the basic body, wherein the second surface is disposed opposite the first surface, and wherein the first sealing ring protrudes from the second surface, wherein the flange has a second sealing ring which protrudes from the second surface, wherein the second sealing ring is provided on the edge of the opening of the basic body, and the plastic cap is integral; b) a bolt with a shaft and a bolt flange, wherein the shaft is accommodated in the receptacle of the plastic cap; c) a metal sheet with a metal sheet opening, wherein the shaft is accommodated in the metal sheet opening such that the plastic cap facilitates the pressing of the metal sheet, wherein the metal sheet has a first side and a second side, characterised in that the first sealing ring and the second sealing ring abut on the second side of the metal sheet. a) a plastic cap with: - a basic body, which has a receptacle, suitable for a screw, with an opening, wherein the basic body extends along a longitudinal axis (Xa) between a first and a second portion, wherein the basic body has an outer surface and an inner surface, the inner surface points in the direction of the receptacle, and - a flange, disposed concentrically to the longitudinal axis, with a first sealing ring disposed on the flange circumference, wherein the flange is provided on the circumference of the opening against the first portion, wherein the flange has a first and a second surface, wherein the first surface points in the direction of the basic body, wherein the second surface is disposed opposite the first surface, and wherein the first sealing ring protrudes from the second surface, wherein the flange has a second sealing ring which protrudes from the second surface, wherein the second sealing ring is provided on the edge of the opening of the basic body, and the plastic cap is integral; - a basic body, which has a receptacle, suitable for a screw, with an opening, wherein the basic body extends along a longitudinal axis (Xa) between a first and a second portion, wherein the basic body has an outer surface and an inner surface, the inner surface points in the direction of the receptacle, and - a flange, disposed concentrically to the longitudinal axis, with a first sealing ring disposed on the flange circumference, wherein the flange is provided on the circumference of the opening against the first portion, wherein the flange has a first and a second surface, wherein the first surface points in the direction of the basic body, wherein the second surface is disposed opposite the first surface, and wherein the first sealing ring protrudes from the second surface, wherein the flange has a second sealing ring which protrudes from the second surface, wherein the second sealing ring is provided on the edge of the opening of the basic body, and the plastic cap is integral; b) a bolt with a shaft and a bolt flange, wherein the shaft is accommodated in the receptacle of the plastic cap; c) a metal sheet with a metal sheet opening, wherein the shaft is accommodated in the metal sheet opening such that the plastic cap facilitates the pressing of the metal sheet, wherein the metal sheet has a first side and a second side, characterised in that the first sealing ring and the second sealing ring abut on the second side of the metal sheet.