Abstract: A tool for preparing piston rings for installation on a piston to be mounted in a bore includes a first cylindrical bore section having an inner diameter equal in measurement to an inner diameter of the cylinder in which the piston is to be mounted. A second cylindrical bore section has an inner diameter smaller in measurement to an inner diameter of the cylinder in which the piston is to be mounted, and the second cylindrical bore section is coaxial with the first cylindrical bore section. A squaring shoulder separates the first cylindrical bore section from the second cylindrical bore section, the squaring shoulder being oriented perpendicular to the axis of the bore sections. The first cylindrical bore section is adapted and constructed to receive piston rings to be mounted on the piston within the cylinder so that the rings are inserted and removed in the shaping process, thus preventing damage from insertion and removal into the cylinder itself.