Electrodymamic machine rotating mass, including for example induction motor rotors, stiffness tuning methods include selective orientation and compression of modular tie rod assemblies into through bores formed in the rotor lamination core outboard of the rotor shaft during motor manufacture, repair or refurbishment. Stiffness tuning enables a motor manufacturer to tune a rotor's rotordynamic stability, and hence the assembled motor's critical vibration speed. Electrodynamic machine rotating mass tuning can be adjusted in response to machine physical design, operational application and manufacturing variation attributes that impact the assembled machine's critical vibration frequency. Thus the present invention offers a systematic, holistic approach to motor vibration refinement through use of a simple kit of modular tie rod assemblies oriented and tightened in a selected array. Rotor stiffness tuning can be tested virtually on computer work stations. Additional actual rotor stiffness tuning can be performed during manufacture.