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Pelican Technical Article:

Front Brake Rotor Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$25 to $120

Talent:

**

Tools:

5mm Allen, punch, hammer

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New rotors

Hot Tip:

Pre-soak rusted bolts in penetrant oil

Performance Gain:

Car stops better

Complementary Modification:

Flush and bleed brake system

Replacing your brake rotors is a very easy job to perform on your Mercedes-Benz W124 and a great way to get started on "Doing it yourself". In general, you should inspect your brake pads and rotors about every 25,000 miles, and replace them if the material lining of the pad is worn down enough to trigger the pad replacement sensor or there is less than a quarter inch of material on the pad. The rotor can also get heat sunk by excessive short term stops and then sitting with the brake pads fully depressed on the rotor. This will cause the rotor to eventually warp and cause a pulsing sensation when braking.

If you ignore the brake pad warning lamp, you may indeed get to the point of metal on metal contact, where the metal backing of the pads are contacting the brake discs.  Using the brakes during this condition will not only give you inadequate braking, but will also begin to wear grooves in your brake discs.  Once the discs are grooved, they are damaged, and there is often no way to repair them.  Resurfacing will sometimes work, but often the groove cut will be deeper than is allowed by OEM specifications.  If your pads are grabbing or pulsing when you come to a stop, there is a very good chance your rotors are warped and in need of replacement as well. Rotors should always be replaced in pairs. NOTE: If your rotors are really grooved you should replace your brake pads as well.

Begin by safely jacking up and supporting the vehicle and removing the front wheel and remove the caliper. Please see our article on jacking up and supporting your W124 along with removing your brake caliper.

There are several types of rotors made for the W124.
Figure 1

There are several types of rotors made for the W124. Some cross drilled and slotted rotors cannot be turned or milled down to give a new level surface and must be replaced when damaged or warped (red arrow).

Inspect the rotor for damage and wear.
Figure 2

Inspect the rotor for damage and wear. This rotor has a small lip developing on the other edge (as they all do as they wear). If you are going to measure the rotor for thickness make sure your measuring device clears the ridge so as to not give you a false reading. Check the rotor surface for cracks or deep grooves or ridges (yellow arrow). If any cracks are found the rotor should be immediately replaced. Light surface rust like that shown is not a problem and will come off the first time you apply the brake.

The rotor is held to the hub by two roll pins (yellow arrows) and a 4mm Allen screw (red arrow).
Figure 3

The rotor is held to the hub by two roll pins (yellow arrows) and a 4mm Allen screw (red arrow). This is a little over kill as the wheel studs actually hold the rotor to the hub once the wheel is installed and torqued.

Remove the 4mm Allen (red arrow).
Figure 4

Remove the 4mm Allen (red arrow). You will probably have to hold the rotor from turning and if you live in a four season climate it is a good idea to loosen this before you remove the caliper. In extreme cases you may need an impact driver to remove it.

Use a punch and hammer the roll pins out the back of the rotor and hub (red arrow).
Figure 5

Use a punch and hammer the roll pins out the back of the rotor and hub (red arrow).

The roll pins are hardened steel; use a hammer and punch to reinsert them when you have installed the new rotor.
Figure 6

The roll pins are hardened steel; use a hammer and punch to reinsert them when you have installed the new rotor.

You can now remove the rotor from the hub.
Figure 7

You can now remove the rotor from the hub. You may have to give it a few taps with a rubber mallet to get it to break loose. Always hit the rotor on the bell or hat surface (red arrow) and not the surface of the rotor.

Before installing the new rotor make sure to clean the mounting flange on the hub so that it gives a good flat and even surface to sit against (red arrow).
Figure 8

Before installing the new rotor make sure to clean the mounting flange on the hub so that it gives a good flat and even surface to sit against (red arrow). Installation is the reverse of removal. If you have opened the brake lines do not forget to bleed the brakes before driving the vehicle.




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