Exchange Brake Pads (Front)

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Page 1 of 2: Removal
Step 1: Remove Front Wheels
        With the car on the ground loosen the five wheel lugs (on both sides) using either your factory wrench or a 1/2" driver with 3" extension and (19mm?) socket.  Raise the front end of the car (refer to front jacking procedure) and remove the wheel lugs the rest of the way.  Remove the front wheels.(Loosen the wheel lugs while the car is still on the ground.)

Fig 1: Loosen the wheel lugs the rest of the way, once the car is in the air.

Fig 2: Brake caliper and rotor revealed.

Step 2: Remove Brake Anti-Rattle Clips
        The brake clips (shown here being reinstalled) are easily removed using a pair of screwdrivers.  Pry one screwdriver between the hub and clip (where my fingers are holding the clip in Figure 3).  This frees the two little catches (see Figure 3) and enables you to use a second screwdriver (wedged against the caliper) to pry the clip towards you and off the caliper.

Fig 3: Brake anti-rattle clip.

Step 3: Remove Caliper Guide Bolts
        Reach behind the caliper and carefully pry out the two small rubber dust caps (see inset, Figure 4) using your fingernails.  This reveals the two 7mm allen-head caliper guide bolts (see Figure 4) which secure the caliper to the car.

Fig 4: Caliper guide bolts and rubber dust caps (inset).

        Remove the two guide bolts.  Use a 3/8" driver with a 7mm allen-head tip.  This tip is available at most auto supply stores for about $5 and may actually be labeled a "disc brake caliper socket".  You may need to give the driver a light rap with your 3-lb hammer to brake the bolts loose but these were not difficult to remove.

Step 4: Remove Brake Pad Wear Sensor (Left-Front, Right-Rear Only)
        Your M3 has two brake pad wear sensors.  The front sensor is located on the left-hand side and the rear sensor is located on the right-hand side.  The front brake pad wear sensor runs from a wiring harness mounted to the inside of the wheel-well, around the strut (and secured by a clip mounted to it) and then back to the caliper.  The small rubber cap on the brake-fluid release valve also serves to route the green sensor wire (see Figure 5).
There are only two brake pad wear sensors - one front (on the left) and one rear (on the right).

Fig 5: Brake pad wear sensor, installed.  Notice how the sensor wire is secured to the caliper by the rubber cap which covers the brake fluid release valve.
        Using a pair of needle-nose pliers, carefully remove the ceramic end (Figure 6) of the brake pad wear sensor from the worn brake pad.  This sensor does not necessarily need to be replaced, but the ceramic tip is fragile so it is a good idea to pre-emptively buy an extra sensor in case you break the old one when removing it.  If you choose to replace the sensor, the far end of it is located in the plastic harness box mounted to the inside of the wheel-well (Figure 7).  On the left-hand side (pictured) this box contains connections for both the brake pad wear sensor (right) and the ABS sensor (left).  To disconnect this end of the brake pad wear sensor, open the plastic box and snap the lower end of the wire out of the slot where it is seated.  Next use your fingers, or a small screwdriver, to depress the plastic clips while pulling the harness apart.

Fig 6: Exposed ceramic end of the brake pad wear sensor.

Fig 7: Harness end brake pad wear sensor (right) and ABS sensor (left).

Step 5: Remove Caliper
            The calipers may be difficult to remove, particularly if your rotors are very worn (in which case they will have a lip).  You can use a flathead screwdriver, inserted into the brake pad wear sensor hole in the pad, to gently pry the pads away from the rotors.  In practice, I found this difficult to do and didn't have too much trouble simply removing the caliper without spending a lot of time on this step.
            Gently pry the caliper (Figure 8) away from the rotor.  The inner pad is connected to the caliper and will come out with the assembly.  The outer pad, shown in the photo, will remain in place.  It is not secured in any way once you remove the caliper so take care not to drop it as you are removing the caliper.  Once the caliper has cleared the rotor, simply lift the outer pad off of the carrier mount.  The carrier mount is a separate piece of cast iron that is secured to the hub with its own two mounting bolts (see Bentley, p. 340-6, Fig. 8).  This carrier need only be removed if you are replacing your rotors as well as your pads.

Fig 8: Use a flathead screwdriver to gently pry the caliper away from the rotor.
            Finally, suspend the caliper (Figure 7) from the strut assembly.  Untwist a coat hanger and thread the wire through both the caliper and the spring.  Ensure that no pressure is placed on the brake hose.  Note: In the photo below, you can see the old inner brake pad which is still clipped to the caliper.

Fig 7: Suspended caliper prevents undue pressure on the brake hose.
           Proceed to Step 2: Brake Pad Reinstallation or replace your rotors, first.

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Page 2 of 2: ReinstallationContinued from Brake Pads (Front), Page 1 of 2: Removal or, if you have just replaced your rotors, from Rotors (Front).
Step 6: Compress PistonIf your old pads were very worn (and of course they are or you  wouldn't be replacing them) you will need to compress the piston in  order to have the new pads clear the rotor when the caliper is  reinstalled.  You can buy a special tool from any auto part store to do  this but I found it very easy to perform this step using an ordinary  wood clamp.
In order to avoid a potential mess, open up the brake fluid  reservoir and use a turkey baster to remove 6-8 ounces of brake fluid  (see Figure 8).  Put the fluid in a clean cup as you will need to  replace it later.  Leave the cap of the reservoir off.
Fig 8: Remove some brake fluid with a turkey baster to avoid spillage when you compress the caliper piston.
At this point it is helpful to locate something that the caliper  assembly can sit on while you work with it.  Matt and I used an  overturned tar bucket for this purpose.  Remove your make-shift caliper  holder (coat hanger) and place the assembly on the bucket, again  ensuring that the brake hose is not stressed in any way.
If you've already removed the old inner pad, temporarily return  it to the caliper.  Carefully center the wood clamp on the old pad  (Figure 9).  Slowly tighten the clamp, easing the piston back into the  caliper.  You should be able to do this with little resistance.  If you  feel too much resistance, STOP, and recenter the clamp.  Continue to  compress the piston until only a few millimeters of the silver piston  can be seen protruding from the caliper.
Fig 9: Use a wood clamp against the old pad to compress the piston back into the caliper.
Note: If you just can't get the piston to compress, here's what you need to do.  Open the rubber bleeder valve dust cap.  While compressing the piston slowly open the bleeder valve.  Have a receptical handy as you will  lose fluid from here.  If you are careful to keep pressure on the pad  while the valve is open no air will enter the system.  If you are not  careful and air gets drawn into the bleeder valve you will need to bleed the system.  I did not have to open the bleeder valve, nor did I bleed  my car's brake system, so if you screw this up you're on your own.

Step 7: Clean Piston (And Rotor, If Not Replacing)Using brake cleaner carefully spray the inside of the piston to  remove any loose brake dust.  Be mindful not to get brake cleaner on the rubber piston seal as it will degrade this part.  Ensure that the slot  that runs around the inner circumfrence of the piston (see Figure 10) is free from debris as this is where the pad clip seats.
If you are not replacing your discs, use the brake cleaner to  remove loose brake dust from these parts as well.  If you are replacing  your discs simply wipe the new parts down to ensure that there is no  residual grease or oil.
Fig 10: Use brake cleaner to remove any loose brake dust from the inside of the piston.

Step 8: Apply Anti-SquealThe cause of brake "squeal" is counter-intuitive.  Brake squeal  is not caused explicitly by the contact of the pad and the rotor.   Rather, brake squeal is caused by micro-vibrations in the pad as a  result of extremely high-frequency "catching and slipping" of the pad  against the rotor.  Application of brake anti-squeal paste or spray is  similarly counter-intuitive.  I'm not exactly certain how these magical  products prevent these vibrations, but they do.  The trick is to apply  the anti-squeal product to the back of the new pads (the clip  side, not the pad material side).  I've seen many people recommend using BMW Plastilube anti-squeal paste (BMW part number: 81 22 9 407 103) but neither of the two dealers in my area carried it.  I bought a canister  of aerosol product from AutoZone and found it very easy to apply and  effective, to boot.
Lay your new pads out on the ground (see Figure 11) and apply a couple of coats of aerosol product to the back of the new pads.  The product I used had a faint blue haze to it and after 10 minutes or so dried to a tacky finish.
Fig 11: Apply brake anti-squeal spray to the back of the new pads.

Step 9: Install New PadsThe new pads are now ready to be reinstalled.  There is no such  thing as a "left" and "right" pad (unlike for the rotors).  That said,  there is an "inner" and an "outer" pad.  The inner pad has a clip on it, the outer pad does not.  Your box of new front pads will contain two of each.  Clip the new inner pad to the piston - ensure that you have the  orientation correct (see Figure 12).
Fig 12: The new inner pad clips easily in place.

Step 10: Replace Caliper Assembly, Wear Sensor & Anti-Rattle ClipUnlike the inner pad, the outer pad does not attach to the  caliper in any way.  Simply hold the outer pad in place with your hand  as you slide the pad / caliper assembly back over the brake disc.  See  Figure 13.
Fig 13: Holding the outer pad in place, slip the pad / caliper assembly back into place over the rotor.
Replace and tighten the two caliper guide bolts (30 Nm, 22 ft-lb).
Reinstall the brake pad wear sensor being careful to properly  route the cable as shown in Figure 14.  The cable goes behind the strut  and then loops back around, through the closed rubber bleeder valve dust cap and back to the pad.  The harness side of the cable snaps in place.  Snap the cover of the harness box shut.  The ceramic end of the wear  sensor presses easily into the new pad.
Fig 14: Properly routed brake pad wear sensor cable.
Replace the anti-rattle clip.  Ensure that the clip "feet" sit on the "outside" (the side closer to the hub) of the caliper carrier.  See Figure 15.
Fig 15: Proper installation of brake anti-rattle clip.

Step 11: Replace Wheel (Don't Torque!)Replace the wheel.  Hand tighten the five wheel lugs.
Fig 16: Replace the wheel and hand-tighten the lugs, only.
Lower the car back onto the ground following the reverse procedure you used to raise it (see Proper Jacking Procedure).  With the car back on the ground torque the wheel lugs (100 +/- 10 Nm, 74 +- 7 ft-lb).

Step 12: Replace Brake Fluid & Pump The BrakesReplace the brake fluid you previously removed and close the  reservoir tightly.  Shut the hood.  Pump the brake pedal several times  to restore pressure to the brake system as the new pads seat.
Congratulations!  You're done!