24/06/2026 10:54 am
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Automotive Q&A
What measurement order prevents a rotor-runout diagnosis from blaming the new disc?
Review the evidence, add a focused answer, and distinguish confirmed measurements from assumptions.
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A new brake rotor showed 0.09 mm lateral runout after installation. The hub face had been brushed, but hub runout and mounting-index position were not recorded. The shop needs a sequence that distinguishes rotor variation, hub error and clamping debris.
What readings should be taken before on-car correction is considered?
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25/06/2026 9:43 am
Measure and record hub-face runout first with the contact point near the rotor mounting radius. Clean the mating surfaces without removing material unevenly, install the rotor with washers and nuts torqued evenly, mark its index and measure rotor runout at the specified radius. Reindex the rotor to see whether the high point follows the rotor or hub.
Also measure thickness variation at multiple clock positions. Lateral runout can create thickness variation over time, but the two are not the same measurement.
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26/06/2026 10:27 am
Using the new sequence, hub runout was 0.05 mm and rotor high point changed when indexed. A small approved index change brought assembled runout within specification. No machining or second rotor was needed, and the pedal stayed smooth.