Abstract: A process and apparatus for centering a pilot shaft which is inserted into a valve guide of a valve seat to be machined, and serves therein as a guide mandrel for a rotary shaping tool. By electronically controlled drive mechanisms, the pilot shaft is supported in a central position by a swiveling ball maintained in a swivelable manner between two clamping shells, and is held between two displaceable clamping plates and centered in the valve guide. To center the pilot shaft, it is moved in two crossing directions by the drive mechanisms through the horizontal displacement of the clamping shells. The clamping shells are successively moved to their two end positions of their two crossing displacement directions. The displacement path lying between these end positions is electronically measured. The drive mechanisms then move the clamping shells into the electronically determined central position of the two measured displacement ranges.