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Pelican Technical Article:

Rear Rotors Replacement

Steve Vernon

Time:

5 hours5 hrs

Tab:

$110

Talent:

***

Tools:

5mm Allen, hammer

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

Brake discs, new pads, new emergency brake shoes (if required)

Hot Tip:

Adjust your emergency brake while you have access

Performance Gain:

Better, safer braking

Complementary Modification:

Replace brake pads, emergency brake shoes, install stainless steel brake lines, or install new wheel bearings

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 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 or simply replaced with a new one. Discs with grooves not only brake less efficiently, but they also heat up to higher temperatures and reduce your overall braking ability. If the rotors have any cracks in them they should be replaced immediately. Always change rotors in pairs (front or back).

The measurements that you take with your micrometer should be made from the center of the disc. It is common for OEM rotors to have the minimum thickness stamped on the rotor hub or check your owner's manual.

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. With the rear rotors, if the parking brake shoes are worn, then you may need to back off the adjustment sprocket in order to be able to remove the rear disc.

The first step is to safely jack up and support the vehicle. Please see our article on safely raising and supporting your W124. With the vehicle in the air remove the rear wheels.

You will need to remove the brake pads (green arrow) and caliper (yellow arrow) to change the rotor (red arrow).
Figure 1

You will need to remove the brake pads (green arrow) and caliper (yellow arrow) to change the rotor (red arrow). Please see our articles on these procedures for additional assistance.

With the pads removed and the caliper safely hung out of the way (red arrow) you will be left with a 5mm Allen holding the rotor to the wheel hub (yellow arrow).
Figure 2

With the pads removed and the caliper safely hung out of the way (red arrow) you will be left with a 5mm Allen holding the rotor to the wheel hub (yellow arrow). Note: never let the caliper hang by the brake line.

Use a 5mm Allen and remove the single retaining screw.
Figure 3

Use a 5mm Allen and remove the single retaining screw. If you live in a four season area this screw may be rusted in place and need some penetrating oil and in severe cases an impact driver. This screw does not contain any measurable force holding the rotor to the hub; the clamping force is provided by the wheel studs to the hub. The screw is only for locating and holding the rotor in place so the holes in the rotor and hub line up. Once you remove the Allen bolt the hub is free and may fall from the rotor so use care.

While the rotor may fall off the hub there is a much better chance that it will be stuck to it.
Figure 4

While the rotor may fall off the hub there is a much better chance that it will be stuck to it. You may need to give the rotor a few taps with a hammer. Be sure to strike the bell or hat of the rotor (red arrow) rather than the ends of the disk. Also make sure the parking brake is OFF when trying to remove the rotor.

Before installing the new rotor make sure to clean around the wheel hub (red arrow) with a wire brush or some sand paper to give the new rotor a clean and level surface to mount flush to.
Figure 5

Before installing the new rotor make sure to clean around the wheel hub (red arrow) with a wire brush or some sand paper to give the new rotor a clean and level surface to mount flush to. Installation is the reverse of removal.

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Comments and Suggestions:
joe4mbz Comments: I see that the time for the Rear Rotor Replacement has been changed from 5 hours to 8 hours in response to my previous comment. Unfortunately, it went the wrong way. I'd expect the rear rotor replacement to be about 1.5 hours, about half the time of the Parking Brake Replacement which remains at 3 hours and is about right.
March 6, 2015
  Followup from the Pelican Staff: I will send your comments tot he web team, again Thanks for checking back and following up. - Nick at Pelican Parts  
joe4mbz Comments: The Rear Rotor Replacement shows "Time: 5 hours". The Parking Brake Replacement shows "Time: 3 hours". How can this be since the Parking Brake Replacement requires that a Rear Rotor Replacement be performed? Time needs fixing.
March 4, 2015
  Followup from the Pelican Staff: Thanks for catching that, I will have the times updated. - Nick at Pelican Parts  

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