Rotors (Front)

Step 1: Remove Wheel, Brake Clip, Brake Pad Wear Sensor And Caliper

Remove caliper as described in Brake Pads (Front), Step 1.

Step 2: Remove Caliper Carrier

Unfortunately the rotor will not clear the caliper carrier, so the carrier must be removed. Unfortunately Bentley fails to mention this and is also annoyingly ambiguous when referring to removal of the caliper itself vs. the caliper carrier, which is a separate part between the steering arm and the caliper. I suggest ignoring Bentley for the moment (sorry Rob).

The carrier is secured to the steering arm using two (16mm?) bolts. Unfortunately these bolts are particularly difficult to remove. Make sure you give each of these bolts a solid squirt of penetrating oil the night before you begin this job. Use your 3/8" driver with trusty braker bar to tackle these bolts. The caliper carrier then falls free.

Step 3: Remove Old Rotor

Although your stock M3 does not utilize a floating brake disc arrangement, the mounting setup is still quite interesting. The brake rotor is sandwiched between the hub and the wheel, all of which have five holes through which the wheel lugs pass. Once the wheel is removed, the only thing holding the rotor to the hub is a small locator bolt (see Bentley, p. 340-7, Fig 9). The locator does not bear a load and is provided simply as a convenience for properly aligning the rotor to the hub. You'll notice that the wheel bolts have an interesting taper (see inset, Figure 1). These tapers serve to ensure that the hub-rotor-wheel assembly is always perfectly aligned. When the brake pads clamp the rotor, the five wheel bolts are completely in shear. The tapering ensures a perfect fit such that all five bolts always bear the load evenly. There is never a lateral force applied to the brake rotor. As such there is no need to affix the rotor to the hub. In fact, even if you lost the rotor locator bolt, it would be of little consequence save a bit of extra frustration when you go to mount the new disc.

Remove the locator bolt using a (5mm?) allen key. If you find that this bolt is corroded, a light tap with the 3-lb hammer should break it loose. If the bolt is really corroded and won't budge, don't force it - it will shear if you're not careful. Use some penetrating oil to loosen the bolt.

The old rotor will likely be corroded to the hub. Working from the rear of the hub, place your breaker bar against the back of the rotor (at the outer edge) and give a quick rap with the 3-lb hammer. The rotor should fall free. Although I can think of better uses for a screwdriver, we were also successful by placing the handle of the screwdriver against the back of the rotor and giving a quick rap to the tip.

Fig 2: Old and new rotors (back)

Fig 3: Old and new rotors (edge)

Step 4: Install New Rotor

Installation of the new rotor is the reverse of removal. Be mindful to install the rotors on the correct sides - there is a left and a right. According to Bentley, "M3 models use directional brake rotors and must be installed on the proper side. Part numbers are cast into the rotors. A part that ends in an odd number is a left rotor. One that ends in an even number is a right rotor."

As a sanity check, note the direction of the fins in Figure 4, below. The installation direction of the fins is a little counter-intuitive. The direction of rotation of the rotors is such that the fins "draw" air, rather than "scoop" it. I was surprised to find that either rotor will fit either side, so this step is not "dummy proof". If you get the installation wrong your car will stop but the rotors will not cool properly and may warp prematurely.

Align the locator hole in the rotor with the locator hole in the hub and tighten the locator bolt (16 Nm, 12 ft-lb).

Replace the caliper carrier (see Figure 5) and tighten the two bolts to 110 Nm (81 ft-lb). Note: I read this value off the table on page 340-6 in Bentley which states, "Front brake caliper to steering arm ... 110 Nm (81 ft-lb)". Of course, the caliper does not mount to the steering arm, the caliper mounts to the caliper carrier which mounts to the steering arm. I made the leap ... proceed at your own risk. In reality there isn't much risk of stripping here - you are securing two cast iron pieces with some serious bolts. None-the-less, don't over-tighten!

You're done! Proceed to brake pads Step 2: Brake Pad Reinstallation.