Abstract: An apparatus for measuring the bore diameter of a hollow member whose bore diameter can change in the axial direction. The bore diameter measuring apparatus includes a probe portion, means for inserting the probe portion in the bore, and means for converting a signal supplied thereto by the probe to a bore diameter indication. The probe portion has a housing structure, a plurality of legs which are radially expandable relative to the housing structure, means for radially displacing the legs, and means for measuring the axial displacement of the radial displacing means. The legs are radially displaceable from a retracted position substantially within the radial outline of the housing structure to selected positions radially therebeyond so as to permit entry of the probe portion through a relatively small portion of the bore and expansion radially outwardly in a large portion of the bore.