Abstract: An improved expansion assembly for an expander tool is provided, and a method for expanding a surrounding tubular body within a wellbore. The expansion assembly first comprises a piston disposed within a recess of the expander tool. The top surface of the piston closely receives a body. The bearing body includes one or more races which house at least one bearing. In one arrangement, at least one bearing is provided which rotationally resides within a single race. In another arrangement, a plurality of bearings are provided that recirculate within one or more races. In this arrangement, the bearing body is preferably a tapered, cylindrical roller actuated into contact with the surrounding tubular by hydraulic pressure applied from within the bore of the expander tool. These arrangements reduce the geometric size of the expansion assembly, affording a larger inner diameter for the hollow bore of the expander tool itself.