Chapter 3 Brake Tools, Shop Equipment, and Service Information 31 Copyright by Goodheart-Willcox Co., Inc. Feeler Gauges Gap gauges known as feeler gauges are thin strips of steel or brass, available in various thicknesses. Th ese thicknesses are measured in thousandths of an inch, or fractions of a mil- limeter. Modern feeler gauges have both thousandths of an inch and millimeter markings. One thousandth of an inch, 0.001″, is about 0.025 millimeters. Steel feeler gauges are used to measure clearances of parts where magnetism is not a factor. Th ey are oft en used to work lip seals, such as caliper and master cylinder seals into place. Brass feeler gauges are used to set clearance on parts containing magnets, such as speed sensors. Brass feeler gauges resemble steel gauges, but can be identifi ed by their distinct golden yellow color. Most brass gauge set sizes range from around 0.004″–0.015″ (0.10–0.38 mm). Feeler gauge strips are usually sold in sets to cover a range of uses. See Figure 3-8. Rotor and Drum Micrometers Th e micrometer is a basic measuring tool for many automo- tive operations. Micrometers are used during brake service to measure rotor thickness and brake drum diameter. Th ese measurements are extremely important, since one of the primary jobs of brake rotors and drums is to absorb heat. If the rotor or drum becomes too thin, it can distort, which may lead to overheating. Measuring with a micrometer will let you know whether the drum or rotor can be turned or must be replaced. Figure 3-9 shows typical drum and rotor micrometers. Many shops are now equipped with electronic micrometers, Figure 3-10. Toothed rotor Brass feeler gauge Sensor Goodheart-Willcox Publisher Figure 3-8. Feeler gauges are needed for checking sensor clearance and other tolerances. Make sure you have both steel and brass gauges. Brake drum Indicator scale Indicator gauge foot A Thimble Anvil Frame Spindle Sleeve Lock B Bear Goodheart-Willcox Publisher Figure 3-9. A—A brake drum micrometer. B—A typical outside micrometer that can be used to measure brake rotors. Handle these precision instruments with care. Rob Kemp/Shutterstock.com Figure 3-10. An electronic outside micrometer. This tool can provide both inch and millimeter readouts.