The mounting surface of the planetary gear (Fig. 1) in the differential on the automobile and the tractor is spherical, and the accuracy of the spherical radius is high, which requires special measuring tools. In addition, there are chamfers at both ends of the inner hole. The length and angle of the chamfer are free tolerances. Therefore, the universal measurement of the spherical radius is also different from the measurement of the general spherical surface. In production practice, we have summarized a method for measuring the radius of a sphere using a flat plate, a measuring rod, a block gauge and a micrometer. This measurement method is suitable for all spherical measurements and can be achieved.