Lesson #3: The Forgotten Spec

One specification that is almost never measured during a brake job is Lateral Run-out.
Virtually every vehicle manufacturer requires the lateral run-out to be at or below .002" before that vehicle rolls out the shop or off the assembly line. That's half the thickness of a US dollar bill of lateral motion allowed! As modern vehicles employ traction control, skid control, and more sophisticated methods of controlling the braking system, the lateral run-out spec becomes even more critical.

• How is this measured?
• A dial indicator
• How do you ensure that the rotor is within manufacturer spec?
• By clamping the rotor to the hub with even torque and measuring with the dial indicator
• If the rotor is out of spec (above .002" in most cases) you have a couple of choices:
1. Phase matching – which means to try the rotor in more positions, or phases, to determine if there is a better match. This requires loosening the nuts and re-tightening each time, and re-measuring. Unfortunately, if you can't find a position where the lateral run-out is below spec, then you will need to employ a different method. In addition, if the rotor needs resurfacing, then you will still have to perform that function separately.
2. By using a Pro-Cut Rotor Matching lathe, you can match the rotor to the hub in any phase in less than 9 minutes and that includes the resurfacing process!