Abstract: In a fixed angle centrifuge having sample tube apertures symmetrically distributed around the spin axis of a rotor body, two rotor body shapes are disclosed at the rotor body top adjacent the openings to the sample tube apertures. These shapes each permit both release of the sample tube aperture cores and maintain the intersection of the top of the sample tube aperture within a plane normal to the axis of the sample tube aperture. A first shape includes placing discrete facets at the top of the rotor body adjacent the sample tube aperture openings. The second, and preferred embodiment, includes generating a spherical surface utilizing a spherical radius which is taken from the intersection of the spin axis of the rotor and the axes from each of the sample tube apertures. In both cases, precise termination of material molding the rotor body occurs at the top of the cylindrical portion of the sample tube aperture. Sample tube aperture core withdrawal easily occurs.