A torsion spring counterbalance assembly having one end of a torsion spring attached to a center support bracket with the other end attached to a torsion shaft around which the torsion spring is pre-loaded with the torsion shaft releasably locked to the center support bracket for allowing the tension of the pre-loaded torsion spring to be released after installation with an overhead garage door to counterbalance the garage door between open and closed positions via lift bands wound about drums mounted at the ends of the torsion shaft and secured to the door bottom by wire brackets. The drums are selectively connected for rotation with the torsion shaft and disconnected from rotation with the torsion shaft to permit adjustment of the tension and resulting torque of the torsion spring. A safety interlock mechanism limits the rotation of the torsion shaft relative to the center support bracket in the event of a malfunction upon initial installation. A manually actuated eccentric cam selectively locks and unlocks the lift band drums for lateral adjustment of the drums on the torsion shaft to properly align the spacing of the drums relative to variations in door width. A sleeve inside the torsion spring maintains the length of the torsion spring at its fully wound position to prevent lateral thrust on torsion shaft end supports enabling a single spring to be used on garage doors of various sizes and weights.