E28 M5 rear brake swap

The idea behind the upgrade is simple; the M5 has the same rear brakes as every other E28, the tiny 284mmx10mm rotors that are barely adequate to control front-end brake dive with the original E28 M5 front brakes (300mmx30mm). Since my M5 already has the 850CSi front brakes (324mmx30mm), the front-end brake dive was even more pronounced. Not only that, those rear brakes looked kind of silly in the middle of the 17" wheels... sort of like every Honda's front brakes ;-)

small E28 M5 rotor shown on top of large E34 540i rotor.

So after a little bit of research into what is compatible with the E28, I found that the largest (almost...) rear factory brakes that are available are the 300mmx20mm vented rotors as found on the E34 M5 and E34 540i. E34 M5 calipers would have been nice (multi-piston), but they are rare and expensive. I found a used set of E34 540i calipers through Zionsville Autosport (1-800-BIMMERS, www.zionsvilleautosport.com) for a very reasonable price.

E28 M5 caliper shown on right, E34 540i caliper on left.

Actually, these calipers came from a 740i but are the exact same part number as the 540i (735i and 750iL brakes will definitely NOT work)... but 740 rotors are different, 540 rotors are required. I purchased new Balo (absolutely the best) rotors from Steve D'Gerolamo (201- 262-0412, www.ultimategarage.com).

E28 M5 rotor shown on right, E34 540i rotor on left.

What's with the "almost" largest? Well, the E34 M5 could be ordered with the "Nurburgring" package, with monstrous 345mm front brakes and larger (not sure of the size) rear brakes. These are so hard to find that they're practically mythical, although I know two people that have acquired them - and they wouldn't sell them to me, dammit!

Installation was easy as the newer brake package is a direct replacement for the original parts. It's the same as doing a complete brake system replacement:

Driver's-side caliper showing two large bolts to be removed.

Removing the same bolts on the passenger-side caliper. These can be quite solidly in place and may require a breaker-bar extension on the ratchet to break free. After that, they'll just come right out.

1) Remove the old caliper via the two large bolts on the back. Leave the caliper attached via the brake line for now, flip the caliper over the top of the hub or use a piece of wire to hang it from the suspension so there's no stress on the brake line.

2) Remove the brake rotor by removing the single 7mm hex bolt. The rotor will then pull right off (or may require a whack with a rubber or brass hammer if rusted in place).
Be very careful not to strip the hex head, it is very soft metal and the bolt may be rusted in place. Spraying it with some penetrating lubricant first is a good idea. Don't get any lubricant on the brake rotor surface or pads if you plan to re-use them.
If you do strip the hex bolt (as I have done), you can drill it out. Once you've gotten deeper than where the head meets the thread, it should break off and you can pull off the rotor and remove the remainder of the bolt with a vise-grip pliers. You should replace the bolt, but it's not entirely necessary... it merely serves to hold the brake rotor in place when you put the wheel back on. The wheel bolts will then hold the rotor perfectly well.

3) Install the new larger rotor and secure it with the hex bolt. larger calipers, and transfer the brake lines.

4) With the new caliper still free, swap the brake line from the old caliper to the new. Use a small wrench to remove the brake line from where it enters the old caliper, rotating the caliper (not the line) to unscrew it. Attach it to the new caliper by rotating the caliper (screwing it on) while holding the brake line in place. Sort of reminds me of that joke, "How many conceited people does it take to screw in a light bulb? One - he just stands there holding the bulb while the world turns around him."

tn_reattaching_brake_pad_sensor.jpg (5428 bytes)

5) Attach the new caliper to the hub using the two large bolts in the same way the original was removed. If you were planning on using brake pads (a good idea for efficient stopping power), you should install them before installing the caliper. Sorry, no picture.

Replace the brake pad sensor on the passenger-side caliper.

6) Bleed the brakes, replace the wheels, and off you go!

I specifically decided not to upgrade to stainless steel lines because I wanted to see how much difference there would be with just the larger brakes... stainless steel lines would add another factor which would confuse the results. I do plan on doing that upgrade at a later date.

Appearance-wise, the rear brakes can finally be noticed, really showing through the wheels. The originals looked like they were embarassed and were hiding! The new calipers are also larger but are proportionately about the same size compared to the rotor as the originals were to their smaller rotors. Picture coming soon.

How does it feel? Well, since I used brand-new rotors (I kept the used pads as they seemed nearly-new), I didn't want to beat the daylights out of them. They'll need a bit of normal gentle breaking-in braking to ensure the surface mates up with the pad properly. I did one slightly-harder stop (couldn't resist) and the car did stay very close to level with the additional rear brake bias, no more of that "tail in the air" feeling. I'll give it a few more miles before I really stomp on it just to be safe.

12/31/99 UPDATE: Now that the brakes are bedded in, the difference is remarkable. Originally I had massive nose-dive under hard braking with the small rear brakes. Now with the big rear brakes, when I hit the brakes hard the car simply pulls down, nice and flat. It's the weirdest sensation, like a big hand is pushing the car straight down. It can bleed off speed so quickly, passengers get sick to their stomach. I'm quite pleased with the results!

Parts list:
E34 540i / E32 740i calipers:
left: p/n 34 21 1 160 399
right: p/n 34 21 1 160 400

E34 540i / E32 740i carriers (2):
p/n 34 21 1 160 384

E34 540i / M5 300mmx20mm rotors (2):
p/n 34 21 1 159 659