Rotor runout leads to rotor vibration by causing a condition known as DTV (disc thickness variation).

To avoid this it is essential when installing new rotors to check runout and the precise tool to do this with is shown below, the EBC runout indicator and gooseneck clamp.

This simple metallic flexible gooseneck can be bent to any shape and quickly locked using the knurled nut and attached using the vice grips to any part of the vehicle body work under the car.

This allows you to clamp the snake tool onto your car, position the dial indicator such that it contacts at right angles to the brake disc, and tighten the gooseneck tool to hold the dial gauge in position.

Set the dial gauge to zero and slowly rotate the rotor by hand measuring the number of graduations by which the indicator needle moves.

If the indicator moves more than 0.005 inches or 0.1mm it is highly advisable to do the following:

First, remove the rotor and rotate the disc by one bolt hole and repeat the dial indicator measurement process.

Also make absolutely sure that the vehicle brake hub is absolutely clean and free from rust, scale and dirt such that the new rotor can mount perfectly against a clean cast iron mounting face on the vehicle hub.

The brake rotor must be secured to the vehicle hub using one of the wheel nuts and a spacer if necessary (to compensate for the thickness of the wheel which is not mounted at this time). For this purpose, a series of metal washers is sufficient and securing the disc with just two diametrally opposite wheel studs, finger tighten and nip up very slightly so as to snug the rotor against the vehicle hub.

Checking the runout of a free unclamped rotor only proves that the rotor itself is true from the factory, it does not prove anything about the geometry of your vehicle itself which is the crucial issue here. One of the major advantages of the Pro-Cut machines is that they provide accurate clamping flanges to hold the disc up snug with the mounting flange in the exact position it will sit when your wheels are replaced and lug nuts tightened.

If you are checking runout with an EBC dial gauge you MUST tighten the rotor down to the hub using the lug nuts and some form of spacer to hold the disc tight and find out what that runout figure REALLY is if you are NOT using the Pro-Cut Machine.

If after all of the above, you still have runout in excess of the figures shown, you must take your vehicle soon after fitment to a Pro-Cut on-car brake lathe centre locations of which can be found on the following links: