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I seem to remember that it has been said you can put the vented discs from an early six series on our E12s. Is this right? And what is involved? Just swapping rotors and calipers? Thanks for the help, Jamie

Yes, this is right.
The things involved:

1. The trailing arms with hubs and bearings from early e24, 633 or 635
2. Brake calipers for ventilated rotors from early e24, 633 or 635
3. Brake discs - sport version, double ventilated, cross-drilled - you can check them in "Walloth&Nesch" - in brakes section - e3(for 3,0 S, Si, L + 3,3 L+Li)/e9(3,0 CS, CSi, CSL).
http://www.wallothnesch.com/e/frameparts.htm
These rear double ventilated discs are only 87.50 Euro now.

Thanks, Alexander.
I thought the whole rear sub-frames were the same up to about mid 1982. I guess my main concern is; do you have to swap out the backer plates, or do the discs and calipers just bolt right up? If it's that easy, next brake job will get the upgrade! Thanks, again.
Jamie

Jamie, you are right - the whole rear sub-frame is interchangeable (early e24 up to May 1982). This is the best way, if you can find the whole sub-frame with all the parts there and swap it.
There are some "benefits":
1. You will have ventilated rear discs.
2. You will have 18 mm rear sway-bar (instead of 16 mm) - this is only for 635 and not for 633.
3. Maybe you'll have some luck and find a 3.07 LSD on that sub-frame (only 635), but it will be usable if you have 3.5 engine.
4. Less efforts doing the swap.

On the last place - it isn't a benefit, but I have to share it - the car will not begin to brake better. For some even better braking - think of front brakes swap. But as I wrote in some previous posts - e12 has good enough brakes for street car.

My way was a little harder - I preferred changing the trailing arms with the hubs and bearings.
I'm not sure of which backer plates you are speaking...
The backing plates for drum rear brakes (like in e12 Euro 520) or the protective plates for the rear discs and the place inside them for the parking hand brake shoes?
Anyway - this isn't of vital importance.
The problem lies in the thicker ventilated rear discs - 272x19 mm instead of 272x10 mm solid. This difference (9 mm) is compensated with additional space between the drive flange (constant velocity joint) and the flange of the side shaft. This offset is from the disc/wheel side. So you can't use ventilated rear discs with a non-ventilated side shaft.
Changing this side shaft is more difficult job for me comparing with changing the whole trailing arms (with all flanges, bearings, plates, even the discs with calipers).

Alexander,
Great information. Thank you. I didn't want to purchase a pile of unnecessary parts. I am actually quite pleased with the brakes, but on pricing out replacement calipers, could not believe the price for the stock units, so was looking for a reasonable alternative. Thanks again, Jamie

Jamie,
Please, understand me right - no doubt - it is reasonable to change the whole trailing arms from an early e24 (635), along with the discs and calipers, but only in that case, if you can find them easily and cheap in good condition.
I don't know much about your case. I don't know - if you have twisted or worn discs (with thickness lower than 9 mm) or bad (blocked) calipers.
Probably, it is the second case. You can renovate your calipers without a lot of effort (in case that the pot&cylinder walls are not worn and rusty). Just buy a repair KIT and do it by yourself, don't waste a "suitcase of money" for a replacement calipers.

DEMIURGE wrote: the car will not begin to brake better. For some even better braking - think of front brakes swap. But as I wrote in some previous posts - e12 has good enough brakes for street car.

Sooo, why would anyone do this?

Sooo, my answer will be very simple.

The e3/e9 and e12/early e24 era was another time.
Almost every possible product from that time was build to last.
Every product was made with "passion and desire" with big claims and aiming too high, often a little bit unreasonable. Please, understand me correctly - I'm a big fan of this method of approach.
And not only built "to last", but with the possibility "to last forever".
This is the reason for me and probably for you to own e12, not e28 or e34 model. A friend of mine has said it in plain words: "The cars stopped to be good and robust , when they began to use "plastic" bumpers". Of course, he was speaking about BMW, but generally it was true for the most European cars.
The e3/e9 were the first models with ventilated rear disc brakes - because BMW wanted to show the right method of approach. The next model - e12 has no rear vented discs, even the M535i model. Even the Alpina B7S with the enormous for that time 330 HP had solid rear discs. (But they have to use even thicker front vented discs - this is another story). The only model after e3/e9 era with rear vented discs was the early e24 - 633 and 635 versions. BMW represented the early e24 as a sport coupe - and possibly they think that for that time they were obliged to make some "sport" additions. I think - it was a bit of inertia of the german engineering. They realize after a while - that it is not vital important, so the old e24 were already with solid discs.
BMWs, like our model has very light tail and heavy front - and the braking force is in time bigger for the front brakes. For that reason was added a brake regulator for the rear wheels - to lower the braking force at the rear axle. I don't remember the exact model of Ferrari, which has the same rear ATE calipers. And Ferrari's tail is much, much heavier. I think, this proves the good braking capacity of our rear brakes (calipers).

And then came the e28 and second generation e24 era.
You know - I'm sure, that instead of the 4-pot front brakes with double brake lines, VERY surprisingly e28 used 2-pots caliper with single brake lines and the things went even worse - some of the calipers became even floating with single-pot.
Even the heavier e34 and e32 have no vented rear discs. I'm not sure, but even M5 has solid rear discs...
Just maybe a ten years ago BMW gave a new meaning to the rear vented discs and began to used them again in some "power" models.

Sooo, my answer becomes not so simple - and that is the reason to stop at this point...

Interesting, thank you for taking the time.

As you touch upon in your write up, it seems that having the front and rear brakes balanced is an important part of good braking. Upgrading the front, or the rear would change this balance and the full benefit would not be had unless you found a proper way to balance the system.

Blaise wrote:Upgrading the front, or the rear would change this balance and the full benefit would not be had unless you found a proper way to balance the system.

Not necessarily and not in this case. E24 vented rotors were either the same, or nearly the same diameter with the same size pistons, the vented rotors simply added fade resistance, not a difference in leverage, so no change in balance would be necessary.

Oh, they do look cooler too, but only with the wheels off.

Mike W. wrote:

Blaise wrote:Upgrading the front, or the rear would change this balance and the full benefit would not be had unless you found a proper way to balance the system.

Not necessarily and not in this case. E24 vented rotors were either the same, or nearly the same diameter with the same size pistons, the vented rotors simply added fade resistance, not a difference in leverage, so no change in balance would be necessary.

Oh, they do look cooler too, but only with the wheels off.

That makes sense.

Great info Alexander & Mike!
Thanx very much. I keep lovin' this website because of guys like you!

Mike W. wrote: E24 vented rotors were either the same, or nearly the same diameter with the same size pistons, the vented rotors simply added fade resistance, not a difference in leverage, so no change in balance would be necessary.

"The truth, the whole truth and nothing but the truth" The vented rotors are exact the same diameter, the braking pads have the exact braking area (they are the same). And as Mike wrote - the only benefit is better cooling of the braking elements (the working temperature during hard breaking is lower, compared with the solid discs). Using your e12 as a daily street car - there would be no difference, only in cases of racing or aggressive type of driving - there is a sense to use rear vented discs.

Mike W. wrote:Oh, they do look cooler too, but only with the wheels off.

They look even cooler, when they are double ventilated - with holes or in the last resort - with carvings. Then, even with wheels on, but preferably with specially chosen for that purpose alloy rims , the "view" is much better.