This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Brake discs (or rotors as they are often called) are a very important part of the braking system. The brake pads rub against the discs to create a friction force that is responsible for slowing the car down. If the rotors become too thin, or develop grooves in them, then their ability to stop the car decreases.
When replacing your brake pads, you should always measure the thickness of your brake discs. If they fall below the specified value for your car, then they should be replaced with new ones. Check for grooves in the rotor, and make sure that you take several measurements of the disc in several different places. This will guarantee you that you get an accurate reading. If the brake disc has a groove in it, then it should most certainly be removed and resurfaced by a machine shop. Discs with grooves not only brake less efficiently, but they also heat up to higher temperatures, and reduce your overall braking ability.
The measurements that you take with your micrometer should be made from the center of the disc. Use the following chart to determine if your rotors need to be replaced.
|Type and Year||New|
|Front Rotor, 911 (1965-68)||12.7mm||11.0mm|
|Front Rotor, 911 (1969-77), 911 Turbo (1976-77)||20.0mm||18.0mm|
|Front Rotor, 911 (1978-83)||20.0mm||18.5mm|
|Front Rotor, 911 Carrera (1984-89)||24.0mm||22.0mm|
|Rear Rotor, 911 (1974-77), 911 Turbo (1976-77)||20.0mm||18.0mm|
|Rear Rotor, 911 (1978-83)||20.0mm||18.0mm|
|Rear Rotor, 911 Carrera (1984-89)||24.0mm||22.0mm|
If you do find that you need to replace your rotors, the process is a relatively simple one. The procedure for the front or the rear rotors is very similar, but for the sake of this Project, we'll look at replacing the rears, which is slightly more complicated due to the addition of the rear parking/emergency brake.
The first step is to jack up the car and remove the road wheel. If you haven't already, remove the brake pads from the caliper. Refer to Pelican Technical Article: Replacing Brake Pads, Replacing Brake Pads, for more details. The flexible rubber brake hose is attached to the trailing arm of the car via a large clip. This clip retains both the flexible line and the hard line that connects to the rear caliper. Remove this clip so that you will be able to remove the caliper without bending the hard metal brake line.
Now, unbolt the caliper from the trailing arm where it is mounted. There should be two 19mm bolts that mount the caliper and hold it in place. After you remove these two bolts, you should be able to slightly move the caliper out of the way of the disc. Exercise caution when moving the caliper around. The hard steel line that attaches the caliper to the rest of the brake system can be flexed a little bit, but too much movement can cause the line to break. Bob Tindel of Pelican Parts recommends using a coat hanger to hang the caliper out of the way. In this manner, you will reduce the amount of stress placed on the brake line. Make sure that you do not let the caliper hang from the rubber brake line, as this will most certainly damage the line.
Once you have the caliper out of the way, remove the two small retaining screws that hold on the brake disc. You will need a large flat-head screwdriver for this task. If these screws are really on tight, you may need an impact screwdriver for the task. At this point, make sure that the parking brake is off. You should now be able to pull the disc off of the hub. If there is any resistance, use a rubber mallet to tap the brake disc off.
If you are having a difficult time getting the disc off, it's probably because the parking brake shoes are stuck on the back of the disc. You need to adjust the parking brake so that it's not gripping the disc. For more information on this process, see Pelican Technical Article: Parking Brake Adjustment, Replacing and Adjusting Parking Brake Shoes.
Installation of the new brake disc is a snap, simply push it onto the hub. Before you install the new disc, take a close look at your parking brake shoes and see if they warrant replacing. If you can see metal on the shoes, or the previous owner had a hard time remembering to remove the emergency brake, then it might be a good time to replace these. After you install the new discs on both sides, you should test your parking brake and adjust it if necessary. Again, refer to Project 50 for more details.
After the new disc is installed and the two retaining screws replaced, simply reattach the caliper, and install new brake pads. Your new rotors should last a long time, and you should see an improvement in your braking after the wear-in period for your new brake pads.
The rear brake discs have a slightly different shape than the front discs. This is due to the need for an inner 'drum' area that acts as the surface for the emergency brake to press against. While the 911 has disc brakes at all wheels, the rear parking brake mechanism is most similar to a drum brake system.
Before you remove your brake discs, it is important to first measure them to see if they need to be replaced. Use a micrometer to perform the measurement. If you use a dial caliper, then you might get a false reading because the disc wears on the area where the pads make contact, not on the edges of the disc. Make sure that you take several measurements in order to compensate for potential low or high spots on the disc.
Removal of the caliper is accomplished by unbolting the two 19mm bolts that mount it to the arm. The caliper can be pushed out of the away, and doesn't need to be physically disconnected from the brake lines. Be careful not to bend the solid brake lines. Instead, remove the clip that holds both the solid line and the flexible rubber line to the trailing arm, and use the flexibility of the rubber line to allow you to pull the caliper out of the way.
The brake disc slides off of the hub after removing the caliper and the two small screws that affix it in place. Keep in mind that the lug nuts that hold on the wheel apply the majority of the force that constrains the disc to the hub. Note that the caliper is pushed slightly off to the side. The two parking brake shoes act on both the inside top and bottom of the rotor.
The new disc can be tapped on with a rubber mallet. Make sure that you have your parking brake shoes adjusted away from the inside drum, or they might interfere with the installation of the disc. New discs may not be perfectly flat, and may take a few hundred miles of break-in to achieve their maximum braking efficiency.