Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairMedium
A technician is performing a disc brake service and notes that the vehicle's brake rotors exhibit excessive runout. What is the maximum allowable runout for most passenger vehicle brake rotors before replacement or machining is recommended?
- A0.001 inches (0.0254 mm)
- B0.0005 inches (0.0127 mm)
- C0.002 inches (0.0508 mm)
- D0.005 inches (0.127 mm)
Show answer & explanationAnswer & explanation
Correct answer: C. 0.002 inches (0.0508 mm)
Excessive rotor runout, typically exceeding 0.002 inches (0.0508 mm) for most passenger vehicles, is a common cause of brake pulsation. This specification can vary slightly by manufacturer, but 0.002 inches is a widely accepted maximum limit. Beyond this, the rotor should be machined (if thickness allows) or replaced.
Why the other options are wrong
- A. 0.001 inches is a very good runout measurement, but not the typical maximum allowable limit before service is required.
- B. 0.0005 inches is too tight; this level of runout is generally considered excellent and well within limits.
- D. 0.005 inches is excessive runout and would almost certainly cause severe brake pulsation; this is far beyond the acceptable limit.
Rotor Runout Limit
The maximum allowable deviation from perfect flatness and concentricity of a brake rotor, typically 0.002 inches (0.0508 mm), beyond which brake pulsation and noise can occur.
- Measured with a dial indicator.
- Excessive runout causes brake pulsation (pedal feedback).
- Corrected by machining (resurfacing) or replacing the rotor.
Memory trick: Rotor's wobble limit: two thousandths, don't forget it!