Abstract: An automatic indexer assembly provides a constant axial and rotational movement of a signal-producing-sensor probe through a zone of a tube to be inspected with minimum signal distortion by means of threadedly connected fixed and movable hollow shafts. A probe tail is clamped to move with the movable shaft as it is driven by a constant speed drive motor and gear reducer mounted on a platform and guide plates which move with the movable shaft along a track parallel to the shaft axis. A water swivel and electrical slip ring also move on a platform with the movable shaft. Twisting of a fluid supply line and an electrical supply line is minimized. Constant sensor movement can be accomplished to permit accurate recording of the signal produced on a recording means with a minimum of backlash, acceleration, deceleration, or intermittent variables distorting the signal.