An expanding lock pin for side entry turbine blades for turbines, including steam turbines compensates for worn or oversize mating pin grooves formed in rotor steeples and blade roots. The expanding pin expands circumferentially to fill the oversize mating groove. In one embodiment of the expanding lock pin a tapered plug mates with a mating tapered aperture formed in the pin. As the plug is driven into the pin its circumference increases. The expanding lock pin eliminates the need to re-machine worn or oversize pin grooves or machine new grooves.