Auto Service Professional

FEB 2016

Magazine for the auto service professional

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32 | February 2016 small amount of pad material to the rotor. If the rotor has no excess runout, the transfer is evenly deposited on the disc face. If the rotor has excessive runout, the transfer takes place unevenly, resulting in higher build-up at the point(s) of higher runout. This results in varying degrees of friction between the pad and rotor (causing a "slip-stick"). The excess, or uneven buildup, can also be caused by improper caliper piston return, rusted or sticking caliper slides or even worn/loose hub bearings. If you see uneven bluing wear on the discs, suspect this uneven pad transfer, resulting from too much runout or the aforementioned caliper issues. Remember: Excessive rotor runout can be caused by worn or loose hub bearings and may not be caused by the rotor itself. Rotor coatings Everyone can agree that a rusty rotor hat looks horrible on vehicles that feature wheels that allow the rotors to be visible. Yes, you can paint the rotor hats with a high temperature paint (paints are available The Chassis Ideally, if rotor disc resurfacing is required, this should be accomplished with an on-car lathe. This eliminates variables such as potential hub runout. Once machined, the rotor must always be installed in the same clock position. If an on-car lathe is used, be sure to place match- marks on the rotor hat and hub to avoid stack-up of lateral runout. Cleaning rotor hat faces by removing rust buildup is critical to promote a fush-mount of the wheel to reduce the chances of lateral wheel runout, but caution is needed. The use of a wire brush is a safe option, while using a power tool and an abrasive disc requires great care, since a too-aggressive approach can cause an uneven surface and resulting runout.

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