Abstract: A rotor adjusting apparatus for adjusting flank clearance between the lands of meshing first and second rotors in a positive displacement screw machine is disclosed which includes a base configured to mount proximate an end of the meshing first and second rotors in the screw machine. A locking mechanism is supported by the base and operatively configured for locking the first rotor shaft in a fixed position relative to the base. An adjusting bar is configured to attach to the second rotor shaft for rotational adjustment thereof in relation to the first rotor to establish clearance between the land flanks of the meshing first and second rotors. A measuring tool is adjustably mounted to the base permitting measurement of rotational adjustment of the adjusting bar. A second locking mechanism is supported by the base and operatively configured for locking the adjustment bar in a fixed position in relation to the base and, thereby, the second rotor in relation to the first rotor.