Abstract: A conventional X-ray tube and shield assembly for use in dental radiography is modified. The conventional assembly has the X-ray tube affixed within an open-ended cylindrical shield such that the focal point of the tube is centered with respect to a small opening provided in the shield. Positioning of this tube-shield assembly within an X-ray generator is then accomplished by trial and error means. The improvement includes an interiorly threaded tubular member extending outwardly from the shield and concentrically around the small opening of the shield. The threaded tubular member is received by a filter element which is disposed within an eyeport opening of an X-ray generator. Since a close dimensional clearance is provided between the tubular member and eyeport opening, the tubular member thus automatically self-centers the X-ray tube and shield within the X-ray generator upon the filter element threadedly engaging the tubular member.