Abstract: In an expansible core, a plurality of first plate segments are moved radially outwardly by means of a conical portion on an operating rod which is moved axially along the longitudinal axis of the core. A plurality of second plate segments are moved axially by the operating rod into the plane of the first plate segments and then expanded outwardly into the spaces between the first plate segments. First and second retraction means are also provided on the operating rod and are engageable respectively with the first and second plate segments to retract the plate segments from their expanded positions upon axial movement of the operating rod in the other direction.