fitting the e28 rear end in to a e21 .

[brockzila]

Hi guys .I'm trying to gather a bunch of info into one place.


I have found info on the swap, but no one that I have found has posted up all the info. I still have/had questions long after I started looking.

I am hoping that this will become one thread with the information/ how to for any one in the future wanting to perform this task.

Why perform this swap.

There are several reasons to perform this swap.

Almost all come down to stgrenth of components. Every single component in the e28 rear end is bigger and stronger than the e21 equipment.

You also have to take in to consideration that it is also the closest to the e21 in terms of design. No/ very little modification to the floor/body work is needed. If using the e30 rear t the fuel tanks and floor need to be removed to allow space.

The main disadvantage to this is the e28 wider than the e28 and you will need to use a high offset wheel , approx 45, or install fender flairs. From what I have been told it's about 30mm per side.

What is needed

Complete e28 rear bar. From my research. Try and get as late model as you can, from the bigger numbers if you can. They have bigger wheel bearings and the later cars have dogbones to help with stabaliasation and extra support. The arms are the same as e34s and some 7 series, but the 7 series will make the track even wider .

E28 rear shocks are a straight fit. Or you can use the e21 shocks and change the bottom bushing as the e28 has a bigger bolt.

I have been told that if I'm using the drive shaft out of a 6cyl it is plug and play.

The e28 hand brake cables can be used but modifications are needed, this I have yet to discover in detail.

If using the e28 diff cover a hole in the floor will need to be cut. I have seen an e30 diff cover used with out cutting the floor, I'm trying to get measurements off a e36 now. E30s have been modified to fit the e36 twin mount cover. I'm hoping to have this info soon .

The sub frame needs the mounts off the e21 cut off and rewelded on to the e28 sub frame. Lots of photos on the net. I will update this with pics soon .

Sway bar . The e28 swaybar will not work due to the spare tyre well. I have seen adjustable race sway bars used and others use nothing at all.

I have not don't this my self. I have all the parts needed and am planning on performing the work in the near future.

If I have made any errors please feel free to correct me, add photos, give more detail if you have some

[ruh]

Hi guys .I'm trying to gather a bunch of info into one place.

That is a great initative. I also seached the internet thin when i did mine, and the info was very scattered.

If you need pic´s i have a bunch too.

Also i can add to the info above that E39 rear coilovers are a straight fit (not from the estate but the sedan) on the car after the conversion.

Unless you buy a practically new E28 rear frame you will want to change the bushes in the sway arms. Those suckers are stuck like a SOB.. E34 polyurethane ones will fit, and are cheap on Ebay. I needed 18 tons of pressure to get this one out!

[brockzila]

Ahhh those are the rear bearings out if a m5. Some have a cylindrical bearing rather than a rubber bushing.

Yeah man you have real experience with this swap so pictures will help.

I'm also sure peeople will want to see the e39 conversion ad well.

[ruh]

I believe that some of the old 6 series (E24) had those cylindrical bearings that was adjustable on a concentric washer.

I have sent you a mail with a link to one of my photobucket albums. Around 100pics are in there, feel free to use any of them you want.

[ruh]

Just found a huge amount of pictures, so i can show you guys how i did mine.

First off, i am not saying that this is THE way to do this. It can be done in many ways, and if i ever where to do it again i´d might do some things differently.

But anyways, i got hold of a E28 525 rear complete with disc brakes etc. It had been lying in some guys backyard for some time. The bonus in that aspect was that it was not covered in oil and dead groundhogs, instead it was very dry, and pretty rusty..



First things firt, i removed the old E21 one:



The only thing you need to keep is the brackets of the cross member. They can bolt off and they can as they are not the same height top to bottom from the bracket, you can actually turn them upside down. On my car i did that because i discovered that the E28 rear sat pretty low to the ground.

I do not take credit for this picture, but this is one way to do it before you cut the E21 brackets off.



I chose to take my E28 rear totally apart and media blast it lightly before paint.



Spend a good amount of time cleaning everything up. Please note that i do not use the drum brakes on the E21, i have hooked everthing up to hydraulics (including the handbrake), so i only use the disc brake function. I have no idea how the E21 and E28 are in regards to the drum brake function!



I also renovated the axles, an here is a word to the wise about those bolts!

When you buy new rubbers for the E28 axles please observe the new bolts that follow the kit. Mine was nice and shiny but they where too short! The threads barely had a hold of the hub!



Also on the head of theese new shiny blingbling allen head bolts are stamped the quality of the bolt. The new ones was stamped 8.8 wich basically means they are softer than butter in the sun! If you are into the ANSI standards of bolts you would know that theese new bolts are about to be a major doodoo.. I am already picturing broken bolts in the diff..

Fortunatly new ones in the right length and in a kick-ass quality is easy to get from any parts and accessories shop. I got theese hardened steel ones instead, stamped 12.9.



Next step was cut and weld. It seems that one the E28 frame the E21 brackets needs to be level to the sway arm bracket.



Polyurethane bushings from a E34 is a straight fit.



Test fit on the car



Once again there are more than 1 way to do this. I chose to use the E28 rear cover for the diff, and that means i had to cut the spare wheel well and make a mounting bracket. That meant i had to remove the originale bracket for the E21 diff.



The part in red line is the cross member of the car, the yellow cross indicates the original bracket for the diff, and that was just cut off.

In the trunk i needed to make a cut



And make some reinforcement



I test fitted some 7 series wheels to see if everything was straight.



And that was practically it.

The prop shaft bolts straight on to the e28 diff.

A few pointers. I have a side exit exhaust on my car, but if you want to keep an original looking exhaust you will need to make your own, as the E28 parts fill the space under the car quite a bit. It is possible, but i chose the easy way.

Also if you have the E21 with the 2 seperate fuel tanks they can still fit under the car, but i have been told by a mate who did this to his that the single fuel tank setup cannot fit.

I was actually suprised at how easy it was. It can be done over a weekend if you have a mate helping out.

The wider track is a issue, and the wheels on the pic are 8x15 ET0 wich is not helping at all.



I chose the easy fix for this problem, and i figured that the E21 is very oversteer happy anyway so to counter that with a little increased track on the rear wouldn´t hurt. But again that is of course a question of taste.





Another option to keep a original look and track would be to use some E36 high offset wheels and use a spacer up in front of the car to get a even track.

Anyways, this was how i did it. As stated in the beginning i am not saying this is the perfect way to do it, i have made alot of compromises and cut quite a few corners in the process.

[Jason_323i]

I did this a few years ago still have the jigs, took a different approach and made the axle solid mounted, with a e36 diff cover for a dual ear mount.

J

[ruh]

I did this a few years ago still have the jigs, took a different approach and made the axle solid mounted, with a e36 diff cover for a dual ear mount.

J

I think you did a thread on it back in the day, that was very inspirering, and actually made me decide to take on the E28 modification

[Jeroen]

Nice detailed write up, is becoming a nice howto like this. Thanks!

[ruh]

Oh just came to think of the front. This is very important too. When you convert the E21 front to 5 stud you can use the E12 hubs. The e12 hubs will bolt directly to your front strut with no problems at all. The e12 and e21 use the same bearings too. So that part is really just plug´n play.

You can also use the E12 calipers (4 pots) from the 528 and those are really nice, vented discs, Ø280 x 22mm, they are a tad bigger than the 323 vented front discs, but it is possible that 13" wheels wont fit on them (! i dont know for sure). Only thing is that the 528 calipers will only bolt on to the E21 323 front struts!!!! This is very important!

Not even 320-6 front struts will take the 4 pots of the e12, so either you need to be very good with a welder and modify your struts, or you need to get some 323 front struts.

[brockzila]

Ok so I did not reply to this as the quality of the answer from ruh was great.

But I have a question.

I have the e21 subframe ears cut off.

I have the e28 subframe cut flush with the mounting tabs. .

How did you know where to mount the brackets on the sub frame in terms of front to back. Get it wrong and the wheels are not in the centre of the arch.

Have you got any closer photos of where you mounted the ears on. It's off center in the photos do you have a measurement for how off centre it is?

Thanks.