During rotor inspection, which instrument would you use to assess runout and thickness variation?

• A. Disc Brake Micrometer
• B. Dial Indicator
• C. Depth Micrometer
• D. Radius Gauge

Answer: (A)

When inspecting a rotor, you want an instrument that directly measures the wear and runout characteristics of the disc. A disc brake micrometer is designed for brake rotors, allowing you to take precise thickness measurements at multiple points around the rotor’s circumference. By comparing those readings, you can detect thickness variation, which indicates uneven wear and potential imbalance or rotor replacement needs. While a dial indicator is excellent for actually measuring runout as the rotor spins, the disc brake micrometer specifically targets the thickness aspect around the rotor and is built for rotor-specific measurements, making it the most suitable choice when assessing both wear distribution and potential runout in this context. The other tools measure different things: a dial indicator focuses on runout but not thickness variation on its own, a depth micrometer measures depths rather than overall rotor thickness around the circumference, and a radius gauge checks shapes that aren’t the primary concern for rotor wear.