e28 subframe upgrade

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I've successfully upgraded my e12 with an e28 subframe, and wanted to share my experiences in case it inspired or helped someone else. As we know, it's a direct swap but the e-brake and swaybar both need to be solved.

Initial pickup



First problem - one of the mounts is badly damaged. I pondered what to do, eventually repaired it by welding in a Garagistic rear subframe camber/caster kit over it and all of the mounts for added reinforcement.



Cleaned and mocked up with the separately purchased e34 trailing arms and cv axles, and e30 diff. I was trying to figure out swaybar solutions.



Painted and installing new rubbers



Out with the old



Pretty clean up there, but the old diff seals were leaking and sprayed



e28 lines up perfectly, as expected



Now time for the e30 diff with early e28 diff cover (center mount). I rebuilt it with an e34 LSD with 3 clutch discs. It's a 3.9:1 which runs out 1st too quickly. A 3.64:1 ratio with the 3.5 would be better.



Subframe and diff installed with new mount. I thought about alternative mount upgrades but I plan to reinforce the stock mount with a plate in the trunk, as others here have done.



It's a close fit, the brake line T piece hits, I had to put in a lower profile bolt. You can see the Garagistic reinforcement plates. I didn't bore out the holes, stock is fine. I'm also not a professional welder.



So the e12 springs did not sit on the new e28 Bilstein shocks. My solution - I cut off the e12 perches and tack welded them in place on the e28 perches and painted them



Springs installed, works great



All assembled. Separately I rebuilt/regreased the e34 cv axles, and also rebuilt and cleaned the e34 brakes

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e-brake cables - I used the e28 cables but the tubes on the e28 chassis are longer and come out behind the subframe, not before. My solution - I measured the diameter of the tubes



9mm, so I ordered some stainless steel tubes with an internal diameter of 10mm. I then measured the distance between the e12 tubes and the e28 ends, cut the tube and made up the difference. e-brake works as normal now.

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Swaybar. I considered using the e12 bar or the e34 bar and welding up mounts on the subframe (as they're both subframe mounted), and a number of other options. My requirement was it couldn't hang low, the exhaust is already too low for my liking and the subframe mount options were all going to be awkward and require removal of the subframe again.

I noticed the e28 has chassis mounts on a 45* angle that I figured I could replicate with a 90* mount. I also noticed it lined up with the fuel tank guard very well.

So I used 90* steel brackets to mount it to the fuel tank guard and reinforce that, then install the same bracket on the other side. Once installed, it had some back/forward movement, so I reinforced it again under the spare tire carrier, with all holes being painted and braced.









The front swaybar is an e12 ST swaybar, and this is an e28 rear. I don't know if they are really 'paired', but the car corners flat. Given the 225 wide tires it doesn't break loose so I don't really know how it affects grip. But it corners confidently so even though it doesn't look pretty it does the job.