Replacing rotors on every brake job is not a cost-effective or efficient way to perform brake service. You may save 10 to 15 minutes by not having to use a dial indicator and a micrometer, but your risk of a comeback can dramatically increase.
Even if you use new rotors, your chance of a pulsation comeback could be greater than if you left the old rotors on the vehicle. Runout in the hub and new rotor can stack up to cause disc thickness variation (DTV) in a few thousand miles. DTV is the main cause of pulsation, so it’s important to check for runout on both old and new rotors following the below procedures.
Sometimes rotor wear can be seen with the naked eye, but, by this time, the rotor is unsafe. If you can see a difference in plate thickness, it is a sign that the hardware and caliper need replacement.
Rotor thickness should be measured with a micrometer in at least six areas on the rotor’s face. In general, the DTV should not vary more than 0.001.” Some manufacturers specify zero DTV.
Runout that’s greater than 0.006” is a sign that the flange, rotor and/or bearing should be replaced. The needle of the dial indicator should be perpendicular to the rotor. Measurements should be taken a quarter inch from the edge. First, find the lowest point in the rotor and set the dial indicator to zero. Then, rotate the rotor to find the high spot. This is the total indicated runout. Mark the high and low spots on the rotor.
Before the rotor is removed, make sure you mark its position on the flange. If the runout is minimal, try repositioning the rotor 180º on the flange to cancel out the runout.
The hub flange is ground zero for pulsation problems. Any runout in the flange will be magnified by the rotor. Before the flange is measured, remove as much of the corrosion as possible.
Do not use a wire wheel. This could damage the flange surface. Some flanges and lug studs have anti-corrosion surface treatments that can be damaged by over-aggressive abrasives.
On some flanges, measuring runout is difficult due to a very small area on which the needle of the dial indicator can ride. Most manufacturers recommend taking the reading as far out from the center of the hub as possible. As a general rule, there should be less than 0.002” of lateral runout.
As much as 0.006” of runout can be corrected with runout correction plates. The notch in the plate should line up with highest point of runout. After the rotor is installed, verify that the runout is below specifications.
