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Pelican Technical Article:

Replacing Brake Discs
on Your BMW

Difficulty Level: 4
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

  This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series.  The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts.   With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
 
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[click to enlarge]

     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 the frictional force responsible for slowing the car down. If the rotors become too thin or grooved, their ability to stop the car decreases.

    
When replacing the brake pads, always measure the thickness of the brake discs. If they fall below the specified value for your car, replace them. Check for grooves in the rotor, and take several measurements of the disc in several different places to guarantee an accurate reading. If the brake disc has a groove in it, it should at least be removed and resurfaced by a machine shop—or, even better, replaced with a new one. Discs with grooves brake less efficiently and also heat up to higher temperatures, further reducing their overall braking ability.

    
Take the micrometer measurements from the center of the discs. It is common for OEM rotors to have the minimum thickness stamped on the rotor hub. If you can’t find this information, use the following chart to determine if your rotors need to be replaced.

Type and Year

Minimum thickness

E36 front solid rotor

10.4 millimeters

E36 front vented rotor

20.4 millimeters

E36 front vented rotor, M3 (1992–1999)

26.4 millimeters

E30 front solid rotor

10.7 millimeters

E30 front vented rotor

20.0 millimeters

E36 rear rotor, solid

8.4 millimeters

E36 rear rotor, ventilated

17.4 millimeters

E36 rear rotor, M3

18.4 millimeters

E30 rear solid rotor

8.0 millimeters

     If you need to replace the rotors, the process is relatively simple. The procedure for front and rear rotors is very similar, but for the sake of this project we’ll look at replacing the rear rotors, which are slightly more complicated due to the parking/emergency brake.

    
First, jack up the car (see Project 1) and remove the road wheel. If you haven’t already done so, remove the brake pads from the caliper (see Project 51). The flexible rubber brake hose attaches to the trailing arm of the car with a large clip. This clip retains both the flexible line and the hard line that connects to the rear caliper. Remove this clip in order 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 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, however—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 small screw that holds on the brake disc (once you’ve made sure the parking brake is off). You will need a 5-millimeter hex head tool for this task. Pull the disc off of the hub. If there is any resistance, use a rubber mallet to tap the brake disc loose.

    
If you are having a difficult time getting the disc off, the parking brake shoes may be stuck on the back of the disc. Adjust the parking brake so it’s not gripping the disc (see Project 53).

    
Installation of the new brake disc is a snap—simply push it onto the hub. Before you install the new disc, though, take a close look at the parking brake shoes to see if they warrant replacing. If you can see metal on the shoes, or if the previous owner had a hard time remembering to remove the emergency brake, then it might be a good time to replace them. After you install the new discs on both sides, test the parking brake and adjust it if necessary (see Projects 53 and 55).

    
Once the new disc is installed, replace the retaining screw, reattach the caliper, and install the new brake pads. The new rotors should last a long time, and you should see improved braking after the wear-in period for your new brake pads.

    
On some late E36 models (in particular, the convertibles), the rear brakes came equipped with rear vented rotors and slightly larger calipers. The theoretical reasoning behind the larger caliper in the rear is due to the heavier weight distribution on the rear wheels of the cabriolet. Although you might think the cars are lighter because they have a cloth roof instead of metal, almost all convertible cars are actually heavier than their coupe counterparts because the chassis is stiffened up underneath. It would appear there is a larger weight bias toward the rear on the convertibles because of this chassis stiffening.

    
Are these vented rear rotors a worthwhile upgrade for the nonconvertible cars? If this is a dedicated track car, then the answer is probably yes. Only add the vented rotors if you’re approaching the heat dissipation limits of the rear brakes. Simply adding vented rotors over solid ones won’t buy you any more braking power under normal conditions where the rotors are adequately cooled. The larger piston diameter of the 328ic caliper might change front/rear brake bias as well, but in general, the stock braking system is designed for the weight distribution of the coupe and sedan. Putting too much braking power on your rear brakes may in fact cause them to lock up sooner, which hurts braking control. Many cars from previous generations are equipped with proportioning valves that limit the braking force applied to the rear just for this reason. Adding the rear vented rotors and calipers would only seem to benefit you if you’re on the track all the time, or if you somehow changed the weight bias of your car.

     If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.  Your continued support directly affects the expansion and existence of this site and technical articles like this one.  As always, if you have any questions or comments about this helpful article, please drop us a line.

Figure
Figure 1
The rear brake discs have a slightly different shape than the front discs, due to the need for an inner “drum” area that acts as the friction surface for the emergency brake. While the BMW 3 Series has disc brakes at all wheels (except for the early 318i), the rear parking brake mechanism is most like a drum brake system.
Figure
Figure 2
Before you remove your brake discs, measure them with a micrometer to see if they need to be replaced. If you use a dial caliper instead, 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. Take several measurements to compensate for potential low or high spots on the disc.
Figure
Figure 3
Remove caliper by unbolting the two 7-millimeter bolts that mount it to the arm. The caliper (green arrow) can be pushed out of the away and doesn’t need to be physically disconnected from the brake line. Hang the caliper from a string (blue arrow) so you don’t put unnecessary tension on the rubber brake line (orange arrow). The caliper mounting bracket (yellow arrow) can be detached from the strut by removing the two bolts on the back side.
Figure
Figure 4
There is a small locator screw that holds the brake disc in place. Use a 5-millimeter Allen wrench to remove this screw, and the brake disc should slide off of the hub. The lug nuts that hold on the wheel apply the majority of the force that constrains the disc to the hub, not this screw.
Figure
Figure 5
Tap on the new disc with a rubber mallet. 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.
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Comments and Suggestions:
Sroor9001 Comments: It'll be same brake pads but only caliper size diffrence and rotor is double from 10mm to 20 mm with gap between them
September 10, 2014
  Followup from the Pelican Staff: Yes, Like I said. The hydraulics will also have to be upgraded. - Nick at Pelican Parts  
Sroor9001 Comments: By the way it's 525 model 2002
This picture of vented rotor not mine
Mine is single disc as in the previous post
September 10, 2014
  Followup from the Pelican Staff: Hot it, thanks. - Nick at Pelican Parts  
Sroor9001 Comments: What benifit from changing from solid rotor to vented rotor
Will it stop faster ??
In e39 for rear not front
I can buy brake caliper and bracket from junkyard for the bigger size rotor
Which is vented
September 10, 2014
  Followup from the Pelican Staff: You will likely increase rotor size and improve brake cooling. Performance overall will increase. Be sure to upgrade the master cylinder as well. Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need.
- Nick at Pelican Parts
 
Sean Comments: Okay - a couple of questions for the experts. One - any tips on how to deal with a sticky locator screw? I'm having the same problem Jon had - can't get the locator screws out.

Question two - has anyone ever heard of a caliper carrier bolt coming free on its own? I took my car into the mechanics a week or two ago, and this past week I started having huge problems with the driver's side front brake - horrible noises, jamming and locking when braking and jerking when resuming motion. I put her up on the jack at a parking lot and found that the lower carrier bolt was missing in action. Seems a little odd that it disappeared so soon after I took the car in on unrelated work I should have been the last person to touch those bolts, and I torqued to about 50 foot pounds when I re-seated them last.

Which leads to my final question - anyone have a link for that specific flange bolt? I'm hoping Pelican carries it, as most dealers and garages around here seem to be out of stock.
April 13, 2013
  Followup from the Pelican Staff: An impact driver can usually get the locator screw out. if that doesn't work, you may have to drill it out.

The bolts will not come loose unless they are under-torqued. Double check the bolt and thread condition inthe steering knuckle. If you give our parts specialists a call, they can help you locate the bolt: 1-888-280-7799- Nick at Pelican Parts
 
jim Comments: John. I'm assuming by "banging the hell out of it" you mean an impact driver? Or hitting your ratchet/cheater? Well, you probably just bought a rotor job at BMW, or some other mechanic you trust. Replacing the pads bought you some time-and that's about it; if your rotors were bad enough you wanted to replace them. Eventually you'll HAVE to replace the rotors you gotta do both, if you just do one.
September 27, 2012
  Followup from the Pelican Staff: Thanks for the Info. We appreciate it.
- Nick at Pelican Parts
 
Brian Comments: Hi sorry to hijack the thread but can't find anywhere else to ask my question.
My young mechanic suggested he fit discs to the rear of my 1970 BMW 2000 which is fine and all works well except after a few brake applications pressure seems to build up and the brakes come on a little not jamming but certainly on it reminds me of a blocked compensating port in older day vehicles.
When the car sits for a time all good again so maybe the fluid is not returning correctly from the rear discs???

Any suggestions welcome
March 12, 2012
  Followup from the Pelican Staff: Could be a build up of heat in the hear brakes because something is binding, check the pads and calipers and make sure everything is ok.
- Nick at Pelican Parts
 
Jon Comments: I give up. I decided that it was time to replace the rotors with this current brake job. I took the first wheel off, grabbed my 5mm hex wrench, and started to remove the small screw holding the rotor on. However, it was obviously stripped and wouldn't budge. I tried, not necessarily in this order: heat, steel wool, banging the hell out of it, a strip of a wide rubber band. I put that wheel back on, took the other one off, and attempted to remove the screw on that rotor. It appeared pristine, if not a little rusty. But it would not budge. I ended up rounding it off. I then took my Dremel and attempted to cut a slit that I could use a large screwdriver on. I then bought a screw extractor, which promptly snapped off in the hole. I gave up trying to replace the rotors and just replaced the pads.

My question: What kind of problems have I just caused myself?
February 20, 2012
  Followup from the Pelican Staff: Don't give up on replacing the rotors, using a impact driver will help get the bolts out, if they are stripped try using a torx socket a little bigger than the hole that is stripped, also using a air hammer and a pointed bit and hitting spots around the bolt on the rotor will loosen it up, and if that doesn't work hit the rotor from behind with a large hammer that will loosen the bolt.
- Nick at Pelican Parts
 

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