Apparatus for circumferentially crimping the annular opposite ends of a cylindrical overtravel spring housing used in a slack adjuster and for maintaining the overtravel spring assembly in assembled relation and an overtravel spring therein in a compressed state. The apparatus includes a three-jaw lathe-type power chuck fixed to a support frame vertically opposite a power cylinder supported on another portion of the frame. The overtravel spring assembly is placed on a post member, fixed concentrically in the chuck, and held fast by the power cylinder when pressurized. With the spring assembly secured in the apparatus, a plurality of crimping blades carried on the jaws of the chuck are actuated to crimp a first annular end of the spring assembly housing, then removing the spring assembly and turning it over in the apparatus to crimp the other end of the housing, thereby securing the spring assembly in assembled relation.