A plug including a hub which extends along a central axis and is defined radially by a side wall wherein an annular groove is formed, which receives a sealing member, the groove provided between a bottom wall and first annular edge wall, and a second annular edge wall, which axially define the groove, the hub being formed by the combination of a first hub section on which the first edge wall is formed, and a second hub section on which the second edge wall is formed, the first hub section and the second hub section being assembled so that they meet at a gasket face which is secant to the central axis and the bottom wall of the groove, so the line of the intersection of the gasket face with the bottom wall is contained axially between the first and second edge wall over the entire perimeter of the groove.