Abstract: The method centers a guide rod, which is inserted in a valve guide of a valve seat to be machined and which serves as a guide rod for a shaping tool. By hand, or with a mechanical, pneumatic, hydraulic or electro-mechanical device, the guide rod is centered in the valve guide by means of a pivoting ball which is mounted in a manner such that it can pivot in all directions between two clamping plates and which is held with the possibility of translation between the clamping ring and the bottom of a housing. For this purpose, the pivoting ball is displaced circularly such that the guide rod sweeps all around the valve guide. By means of the follower levers, the shape of the valve guide is determined and, by means of a precise mechanical device, the pivoting ball is placed at the center of the swept shape, where it can be fixed by means of plates and of the clamping ring.