In the comparator, the specimen is kept with its cylindrical walls touching the four nails protruding in the comparator and its bottom on the other nail coinciding with the center of the bottom end. The plate which caries the dial gage, slides over the flat guide and by this tip of the dial gage is made to scan the whole upright face of the specimen. The maximum dial gage reading gives the deviation of the specimen end from smoothness. When the tip of the dial gage is made to scan the end surface along a diameter, the reading gives a measure of the deviation of end surface from being perpendicular to the specimen axis in the plane containing that diameter and the specimen axis.