Expandable type press plunge (4) comprising two plunge parts, named formation part (8) and expansion part (9), which are axially movable in relation to each other, and in which the expansion part (9) is adapted to provide an expansion of some part of the plunge (4) when said two parts are moved together, and in which the expansion part (9) comprises an expansion disc (19) made of an elastic or deformable material which is cup-formed in its non-operated condition, and in that the formation part (8) is formed with a bottom plate (16) having an upper surface supporting and guiding at least a part of the bottom surface of the expansion disc (19), and in which the outer peripheral surface of the bottom plate (16) is of the same shape and at least the same size as the outer peripheral edge of the expansion disc (19) in its non-operated condition, and whereby the cup-formed expansion disc (19) is more or less flattened and the outer peripheral edge thereof is widened outside the bottom plate (19) when the formation part (8) and the expansion part (9) are moved axially in a direction towards each other.
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