fig1 shows a plastic part 1 e . g ., part of a plug board into which the contact pin 2 is molded . the contact pin 2 is taken through the plastic part 1 so as to be water - tight ; to this end , two truncated cones 3a and 3b are disposed on the contact pin inside the plastic part 1 . each of the two truncated cones 3a and 3b has its major base surface pointing away from the meridian plane 4 of the plastic part 1 and is directed towards that surface of the plastic part which is closest to it . each truncated cone is fixed to and merges at the opposite end into a cross - section of the contact pin 2 . the aperture angle α of the truncated cone , i . e , the angle between the major base of the truncated cone and of the conical surface , is preferably of the order of 70 °. the major base of the truncated cone forms a shoulder 5 extending concentrically around the contact pin axis . the shoulder 5 is cast sufficiently deeply in the plastic material for the plastic layer between the shoulder 5 and the corresponding surface of the plastic part in this zone to have a thickness d which is sufficient to prevent the plastic material from tearing open on shrinkage of the plastic during cooling . this shrinkage , which is represented by the broken - line arrows in fig1 occurs on cooling of the cast plastic . since this shrinkage takes place to a far greater degree than the shortening of the metal contact pin during cooling , forces occur at the interfaces between the contact pins and the plastic material , and will be considered in greater detail hereinafter . the shrinkage movement is in each case away from the surfaces of the plastic part towards the central region thereof , indicated by the meridian plane 4 in fig1 . in other words , a force directed towards the meridian plane 4 acts in each case on the shoulders 5 of the truncated cones 3a and 3b and ensures that the contact pin is retained without play . between the planes of the shoulders 5 and the meridian plane 4 the shrinkage tends to lead to the formation of cavitie 6 around the conical surfaces 7 of the truncated cones 3a and 3b . this effect is not entirely inevitable , although the material shrinkage in the radial direction around the contact pin axis substantially reduces the degree of cavity formation . however , this has no significant effect on the tight fit of the contact pins since it is basically the edge 8 of the shoulder 5 which determines the tight anchoring and sealing properties of the bushing . this edge 8 remains tightly and sealingly enclosed by cast plastic even after shrinkage thereof . the arrangement of the truncated cones according to the invention as described proves superior to other known arrangements and also to simply cylindrical cross - sectional changes . if the truncated cones according to the invention were replaced by cylinders , cavities would form at the shoulders situated towards the meridian plane 4 and could no longer be substantially compensated for by radial shrinkage , so that a far weaker hold would apply for the contact pins axially . if the truncated cones were arranged with their apices each pointing away from the meridian plane , this would also result in large cavities forming inside the plastic part , and the peripheral edge of the shoulders 5 , which governs the sealing properties , would be in the immediate vicinity of such cavities . in such cases , it would not be possible to expect the plastic material to be tightly pressed onto the conical surface of the truncated cones , so that there would be no assurance of either a secure fit or absolute water - tightness . fig1 shows another cylindrical change of the cross - section of the contact pin 2 in the region of the meridian plane 4 . cylinder 9 is fixed to the contact pin and thus serves further to strengthen the holding of the contact pin . its arrangement on the meridian plane 4 ensures that no cavities form anywhere , so that it is tightly enclosed on all sides . thus the tearing - out forces required to release a contact pin from its anchorage are greatly increased . at the same time , its construction and its central position in the molding prevents any accumulation of material . fig2 shows an alternative embodiment of a contact pin in which the waterproof contact pin bushing can be used . this contact pin has a rectangular cross - section . the truncated cones 3a and 3b , however , are rotationally symmetrical , this being absolutely essential to achieving a water - tight bushing . the circular or , if required , oval shape of the peripheral edge 8 of the shoulder 5 for the first time allows sealing shrinkage of the plastic material on to this edge . edges which have a contour with corners have hitherto not resulted in a really sealing - tight contact pin bushing . since only the contour of the peripheral edge 8 is involved , the remaining cross - section of the contact pin may be of any desired shape , more particularly square or rectangular . the peripheral change in the form of truncated cones or cylinders can be obtained by upsetting , hammering or other shaping . fig3 shows a specific example of a water - tight contact pin bushing in a plug board . like elements in fig1 and 2 have been given like references in fig3 . the plastic part 1 is part of a plug board , contact pins 2 being disposed in two rows parallel to one another . the contact pins 2 in the cast plastic part have cross - sectional changes in the form of truncated cones 3 and a cylinder 9 . the truncated cones 3 are each so aligned that their apex points towards the meridian plane 4 of the plastic part and the major base is directed towards the nearest side wall . the cylindrical widening of the cross - section of the contact pin 2 is disposed in the meridian plane 4 between the side surfaces . the top and bottom contact pins 2 in fig3 differ in that their diameters are different on the plug side . in both cases , the diameters of the shoulders 5 and of the cylinder 9 are substantially equal to the diameter of the contact pin on the plug side . in addition to the frusto - conical cross - sectional changes , another cylindrical widening of the cross - section is provided in the case of the top contact pin 2 remote from the plug side , one end face of the widened portion terminating flush with the surface of the plastic part . the material thicknesses between the shoulders 5 and the nearest surfaces of the plastic part are so dimensioned as to reliably prevent any tearing out . the provision of four truncated cones and a cylinder 9 disposed in the meridian plane reliably assures the sealing tightness of the contact pin bushing in the plastic part and provides the highest possible resistance to tearing out . the plug board shown in fig3 can be produced in one operation in an appropriate injection molding tool , from which the finished end product can be immediately removed . no further processing or additional sealing by uv - hardening silicone is required .