Abstract: A water-tight contact pin plug assembly in cast plastic parts (1), more particularly plug casings. The contact pins (2) have cross-sectional variations over their cast part length comprising at least two truncated cones (3a, 3b) which are substantially rotationally symmetrical relatively to the pin axis and the major base surfaces of which form shoulders (5) respectively situated opposite the nearest surface of the plastic part. The distance d between the shoulders (5) and the associated surfaces of the plastic part (1) is large enough to prevent the parts from breaking open in the shoulder zone.