There used to be a rumor that the differential cover from the new M-Roadster/Coupe would bolt on to an E30 M3 differential housing. Jimmy Pettinato was the first enthusiast to actually go out and tackle the install. I have since followed suit, as have some other list members. This write-up is a combination of the experiences gained by Jimmy and I during our install procedures. You may be wondering; "Why the heck would I want all those fins hanging down under my rear end?" Well if you track your E30 then that limited-slip clutch pack will generate some serious heat. And the ultimate enemy of any clutch is heat. The fins on the M-coupe diff' cover reach down into the air flow underneath the car and should therefore help bring about lower operational diff' temperatures. Furthermore, since the diff' cover only costs about $140 and can be ordered right from the dealer, I cannot see any reason not to do it - this mod should be on every E30 owner's "to-do" list.

M-Coupe Differential Cover Here Jimmy reports how his original installation went:

To finalize any doubts... the M-Coupe diff' cover is an absolute, slam dunk, just shy of an hour, bolt in. No mod's of any kind are required, nor is pulling the diff'. Just remove the "ear" mounting bolt, lower the diff' down with a jack, take out all the cover bolts, remove the stock cover plate and replace it with the new one. Raise it back up with the jack and re-stall the mounting ear bolt. Took me under an hour myself.

Jimmy P.

» M-coupe Diff' Part Numbers:

33 11 2 228 335 M Coupe diff'cover $140.28 (net) (required)
33 11 1 210 405 gasket $1.65 (net) (required)

33 11 1 212 682 (x2) drain and fill plugs $0.44 (net) (optional)
07 11 9 963 355 (x2) crush washers $0.44 (net) (required)

» M-coupe Diff' Cover Bolts:

The bolts needed for the new cover are different lengths. Jimmy went out and sourced his own bolts as follows:

All bolts 10 mm diameter @ 10.6 hardness (grade)
2 @ 25 mm
2 @ 75 mm
2 @ 90 mm
No special thread is needed
All bolts are coarse thread (1.5 mm thread pitch)

A good place to get these bolts might be from Steve D' at the Ultimate Garage, as they may be difficult to find locally.

Alternatively, you can order the factory bolts at the same time that you order the M-Coupe diff' cover. This is what I did. The factory bolts are all grade 8.8 and the 90mm bolts that Jimmy used are 85mm long from the factory (both lengths should be fine). The BMW part #'s for the factory diff' cover bolts are as follows:

07 11 9 914 114 (x2), 10mm x 25mm, $0.20 ea.
07 11 9 921 541 (x2), 10mm x 75mm, $0.84 ea.
07 11 9 921 522 (x4), 10mm x 85mm, $1.08 ea.

* Note that these new M Coupe diff cover bolts require a 16 mm wrench/socket (as opposed to a 17 mm size for the existing E30 M3 bolts). Make sure you have the right tools before you start the job.

The M Coupe diff' cover comes with a new bushing already pressed in. This bushing is completely different from the one that the E30 M3 originally came with. It appears to be quite a bit stiffer - an observation which is confirmed by the additional harmonics that have been added to my car under "off-throttle" conditions.

Notice how bad my old E30 M3 bushing looks. It is certainly "long-in-tooth". And this bushing has only 55,000 miles on it. Nonetheless, 10 year old rubber is 10 year old rubber. Also note the way both bushings use aluminum inserts to help provide stiffness, but in different ways. The vertical ridges along the sides of the M Coupe bushing are actually curved aluminum plates. But they get covered with rubber during molding. The E30 bushing (I do not believe that there is a unique E30 M3 bushing) also has these plates - but only one per side vs. two. Keep in mind the forces that the diff' bushing must cope with. Vertical forces are applied during braking and acceleration and lateral forces are produced during cornering. Both bushings are obviously "tuned" to react differently to braking, acceleration and cornering.

The M-Coupe bushing has two fat rubber bumpers both above and below the metal insert. These make the bushing stiffer during acc/braking than the E30 bushing. This stiffness can be "heard" in the form of increased resonance during off-throttle coasting. I am not sensitive enough to actually "feel" it just driving on the street.

Should anyone want to try just the M Coupe bushing as an upgrade, the part number is: 33 17 2 228 425

M-Coupe Differential Cover Here we see the two diff' covers side by side. The M-Coupe cover has much more extensive fins, mostly down below the differential, where the fins protrude into the air flowing below the car.

Note the 10mm hex size on the old E30 drain/fill plug vs. the 14mm hex size on the new M Coupe drain/fill plug. Be sure to have the correct size on hand if you decide to go with the newer style plugs. I got nice stubby 14mm and 17mm (for the tranny) hex head sockets from Steve D' at the "Ultimate Garage".

The red arrows point out the vent outlets for the two types of differential covers. The E30 has a little vertical tube, which would actually make it very easy to attach a hose and run it up to an overflow/catch tank in the trunk. The M Coupe cover has only a small pin-hole. However, the M Coupe cover has a nice baffle on the inside (shown on a different page) which the E30 cover does not have.

This is the inside of the M-Coupe differential cover. The main feature here is shown by the red arrow. This is the baffle that covers up the vent pin-hole and prevents diff fluid from spilling out of the differential casing during spirited driving. I am not sure yet how well it will work. But the standard E30 cover did not do a very good job of keeping the fluid contained during track events so I have little to lose. The baffle is steel and is riveted onto the cover. I sincerely hope the rivets do not decide to let go some day because that would in all likelihood spell the end of my ring and pinion.

M-Coupe Differential Cover Someday I would like to remove the baffle and drill and tap the pin hole for a nice barbed fitting (maybe 3/8" ID) so that I can run a hose to a catch tank.

In case you were wondering what you might find inside the differential, here is a peek. The ring gear is on the left (the pinion gear is around the front side of the differential). The clutch pack is housed in the metal cylinder near the center. The propeller like thing on the right is the trigger ring for the speedo sensor. Do not accidentally tweak it.

And finally to confirm what we already new from Jimmy P. - yes the cover fits just fine. It actually does not hang down as far as you might think at first. The exhaust is still the lowest point on the car.