Abstract: An elevator bail with elongated slots, an elevator tilting assembly and indexing apparatus, and a top drive drilling unit in combination with such devices that are connected to the upper end of a drill string and movable upwardly and downwardly with the drill string along a guide apparatus and that can be provided with a pipe handling mechanism for lifting and positioning the drill string which can can be swung about a vertical axis to a more advantageous attitude for loading or unloading drill pipe. Movement about the vertical axis can be achieved through the use of an air or hydraulic cylinder. Fitted about the top drive central shaft can be a rotable air conducting sealing gland and about its perimeter an annular ring containing a target material that is sensitive to a proximity switching device. The annular ring is rotable through an infinite angular travel and is able to be secured against unwanted rotation.