Abstract: A device for measuring the average distance between the ball retainer seating rings of a ball valve comprises a flat locator plate adapted to fit into the bonnet recess of the valve body, and a downwardly depending support arm attached to the flat plate. The support arm has a first surface perpendicular to the bottom surface of the plate and a second, opposing surface which is at an angle greater than 90 degrees relative to the bottom surface of the plate. The support arm is attached to the locator plate in a position such that when the locator plate is positioned in the bonnet recess the first surface coincides with the axis of the valve body. In one embodiment, probes are threaded and extend from threaded holes in the depending arm in a direction normal to the second surface of the arm. Thumbwheels attached to the probes are utilized to adjust the probes. In a second embodiment each probe is attached to a spring loaded probe drive element within the depending arm.