Abstract: An apparatus for gauging the diameter and concentricity of a bore in a workpiece, the bore having an inner surface, the apparatus comprising a base, a first surface engaging member supported by the base for movement along an axis and relative to a reference point, a selectively adjustable second surface engaging member supported by the base for movement along the axis, the second surface engaging member and the reference point defining the diameter of a circle, a third surface engaging member movably supported by the base and located on the circle, the third surface engaging member moving in response to movement of the second surface engaging member to maintain location of the third surface engaging member on the circle, the first, second, and third surface engaging members being adapted to be housed in the bore and to engage the inner surface of the bore, and the first surface engaging member being biased away from the second surface engaging member and against the inner surface of the bore, and a gauge for