Which tool is used to measure lateral runout on a brake rotor?

• A. Dial Indicator
• B. Micrometer
• C. Caliper gauge
• D. Ruler

Answer: (A)

Measuring lateral runout on a brake rotor requires detecting tiny, precise lateral displacement of the rotor surface as it rotates. A dial indicator is designed for that purpose, giving high-resolution readings of small linear movements. You mount the indicator so its probe contacts the rotor surface and rotate the rotor to observe the dial’s sweep; the difference between the maximum and minimum readings shows the runout. This setup captures how far the surface deviates from a true circle during rotation, which is exactly what you need to assess wobble.

A micrometer measures thickness or small dimensions at a stationary point, not the rotational wobble of the rotor. A caliper gauge can measure overall outside dimensions but isn’t sensitive enough or set up to quantify cyclic runout as the wheel spins. A ruler is too crude for the precise measurement required. The dial indicator provides the necessary precision and the proper method to quantify lateral runout in thousandths of an inch or hundredths of a millimeter.