The dimensions of a portion of an injection molded hollow product formed in a mold cavity between a first cavity surface of a cavity section and a first core surface of a core section are adjusted by adjusting the position of the first cavity surface relative to the position of the first core surface so that the first cavity surface and the first core surface are in a selected relative position when the cavity section is combined with the core section and a protractible and retractable core part is protracted so that a cavity abutment mold part can be connected directly to the protracted core part and the cavity abutment mold part also can be connected indirectly to the protracted core part by a series consisting of abutting mold parts. This series includes a pair of abutting rotatable eccentric rings, which are rotated to adjust the relative positions of the first cavity surface and the first core surface.