The invention is directed to apparatus and a method for determining the diameter of an object and comprises a measuring apparatus having a frame adapted to receive an interchangeable sensor assemblies and a centering head adapted to engage then object being measured. The interchangeable sensor assemblies include a sensor probe that is biased in an outward direction therefrom so that its tip end contacts the measured object, and the probe includes means to generate a signal indicative of its tip end position. The frame portion of the measuring device also includes a microprocessor capable of receiving the sensor probe signals, and the microprocessor is programmed to provide an output display that corresponds to the diameter of the object. The diameter of the object being measured is determined by storing in the microprocessor a calibrated gap distance in the based upon a first sensor probe signal indicative of probe tip position, calculating a measured gap distance based upon the stored calibrated gap distance and a second first sensor probe signal indicative of probe tip position, and determining diameter of the object based upon the calculated measured gap distance.