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This month, Brian Newell looks at measuring disc run-out.

Q: I am about to undertake a periodic ATA assessment. How and where do I take the measurement for disc run-out?

A. Disc run-out is measured using a dial gauge, preferably with a scale of 0.01mm. This needs to be held in a clamp, on an adjustable arm. The arm can be held in position either by a magnetic base or clamp.

The contact tip of the dial gauge needs to be in contact with the disc, approximately 10mm in from the edge of the disc. The disc is then rotated on the hub and the run-out is measured by recording the largest deflection on the dial gauge. The maximum limit for disc run-out is 0.15mm.

When measuring thus - on the vehicle - run-out can be influenced by the condition of the hub and cleanliness of the mounting surface. Any dirt or rust can lead to run-out. You are measuring this at a distance from the centre, so unevenness here will be magnified at the disc extremes.