A non standard thread of pitch 3.175 mm is to be cut on a lathe having lead screw of pitch 6 mm. A change gear set provided with the lathe has one gear each with the following number of teeth: 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 127. The correct pairs of change gears (a/b x c/d) for machining the given thread are (assuming that the transmission ratio of the rest of the kinematic train between the lathe spindle and lead screw is equal to 1),