A vibration isolator assembly, such as an isolator bushing or cradle mount, includes a housing and an isolator connected to the housing. A shaft assembly includes first and second mating components, the first component being connected to the elastomer and having a cavity of a first dimension for receiving the second component having a different, second dimension therein. The differing dimensions alters the stress characteristics of the vibration isolator assembly. According to a preferred method of altering stress characteristics of a vibration isolator assembly, the method includes the steps of forming a cavity in the shaft and varying a dimension of the shaft to alter the spring rate characteristics. In the preferred arrangement, the shaft assembly includes a split, first component and a second component is inserted between the split portions to relieve tensile stress in the isolator and, if desired, to impart a compressive stress in the isolator.