Abstract: Piston blank for a piston, comprising a piston lower part, which comprises a first joining surface running around a central axis of the piston blank, and a piston upper part, which comprises a second joining surface running around the central axis and an inner surface running around the central axis and adjoining the second joining surface as viewed along the central axis, wherein the piston upper part can be placed with its second joining surface on the first joining surface, and wherein a tangential plane which is assigned to the second joining surface is inclined relative to the central axis such that the tangential plane intersects the inner surface.

Abstract: Apparatus for opening and spreading the legs of a seam joining two fabric parts, preparatory to the pressing of the legs of the seam, comprising a first rail having an edge against which the legs of the seam are to be guided, a second rail parallel to the plane in which the pressing is to be effected and having an edge against which the sewn together fabric parts are to be tensioned, the edge of the first rail being spaced from the edge of the second rail toward the center of the second rail by a distance at least equal to the length of one of the legs, the edge of the first rail extending at an angle of from 60.degree.-90.degree.