17 Now you need to set backlash. This is the amount of slack or play between the ring and the pinion gears measured in thousandths of an inch. This is where a dial indicator comes in. Mount the magnetic base on the housing, then set the dial so that it engages a tooth on the ring. Lock the pinion in place (we wrapped a rag around the companion flange several times and held it tightly) and rock the ring gear back and forth to take a measurement. There is usually a small acceptable range (0.005-0.007 inch in this case) for backlash, but it's important to get it right for an accurate pattern.