E28 rear axle setup on E21 and 5 lug conversion.
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  1. #1
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    E28 rear axle setup on E21 and 5 lug conversion.

    Build guide update 05/04-2015

    Hi guys.

    This is a DIY guide that has been missing on the internet for ever. As i am doing this conversion on my own car, i thought iīd might as well document it for others to use.

    The plan

    There are many reasons for wanting a stronger rear axle setup on the E21. For one, the axles themselfes are very small, and will not take much abuse. Some are lucky some are not, i have read about people having to change the axles almost weekly only on account of having a M20B25 lump up front.

    So what will you gain with this conversion:

    1. Bigger and stronger axles that will take almost any engine conversion or tuning option you could think of.
    2. Access to the big diffīs stock on most E28īs are 188mm ones, but you will gain access to newer rear diffs like from E36īs, or the big 210mm ones.
    3. Rear disc brakes are stock on many E28īs, however some uses drums as well. The rear disc conversion can be made using E21 323i parts too but the wheel mounting will then remain 4x100pcd.
    4. 5 Lug conversion as the E28 uses the 5x120pcd.
    5. A all together bigger, more strong construction than the E21īs original rear setup.

    The E28 rear axle conversion is also the easiest way to get a proper 5 lug conversion done if you donīt fancy using adapters. However, if you are only interested in a 5 lug pcd i strongly recommend using spacers. The e28 rear axle setup will add roughly 1 inch (25mm) to the width of the rear wheels. Unless using rims with very low ET / backspace you will need to do something to the rear arches of the car. A 5x120/4x100 adapter will take up the same amount of space.

    The downsides:

    There are however downsides to this conversion. As mentioned the wheel base will change as you will gain a wider rear axle and wheel mounting by roughly 1 inch per side (25mm). This will reduce wheel selection especially the ones with low ETīs and a fat lip. Stock E36 high ET wheels will fit perfectly on the rear. They will however look pretty slag on the front end... 20mm spacers for the front should be considered either way. If you are building a widebody E21 this will never be an issue.

    Front axle conversion

    On the front end of the car the conversion couldnīt be more simple. E12 hubs will fit the e21 struts as long as the e21 struts NOT are pre 1980 (i have been told). Any e12 hub will do, and the E12 and E21 share the same front bearings. It is 100% plug n play.

    E12 front hubs with new bearings ready to go onto the car



    Front Brakes

    The front brakes is as easy as the hubs. Still using E12 parts the "big" E12 528i came with 280mm vented front discs and 4 pot calipers. The parts are readily available as both used and new parts and are not expensive or hard to find. The 4 pot calipers will need a T union piece as it has 2 brakeline feeds, but that is very easy to fix. The E12 4 pots fits the E21 brake bracket perfectly.

    E12 280mm front discs mounted.



    The E28 rear axle setup

    This is not as easy as it may seem, or look on other threads covering this modification. I tend to make the most detailed information around this subject available to others with this thread, so as my work progresses i will change this thread.

    This is my E28 rear axle, it originates from a E28 525i and has the rear disc brake system. It also has a connector for the electronic speedometer in the diff rear cover.

    -img_20150208_131630-jpg

    A couple of important notes. The E28 rear axle setup will not fit the E21īs base as the E28 beam is too wide. So this will need to be shortened and welded. You will also need to re-use the mounting ears from your E21 beam. A final thing you may notice is that the E28 has a diff mount on the diff rear cover that is placed in the center and looks nothing like the E21īs original one. This is a relative easy fix and can be switched by using a cover from a E30. However, nomatter how you go about this you will have to do some welding on the car. The original mount in the bottom of the car is for the small diff case, and needs to be moved nomatter what. In this example i choose to keep the E28 way, and make mountings for this setup on the car by cutting and welding the spare wheel well.

    I figure this is the best way to show how it is done. So bare with me.

    First off you will need to dismount the E21 rear axle. You will also need to remove the exhaust and the gas tanks. If you have a pre 1980 car you will need to get another gas tank as the original one will not have room after the E28 rear axle is in. The "aft" E21īs has 2 seperate tanks connected by a tube in the lower part. They should both fit after the conversion.

    On the E21 beam you will need to cut the "ears" off.



    On the E28 beam things are actually very very simple regarding cutting. You need to cut like shown on the drawing below. This will make it fit PERFECTLY.

    -untitled-png

    Here it is shown from another E21 guys who has done a similar conversion.



    The cut must be completely flush on the trailing arm bracket.

    The next step involves cutting in the car. If using a E28 differential rear lid like me you need to cut a hole in the trunk, like so:



    You will also need to cut and remove the original differential bracket located in the driver side of the spare wheel well as this will be in the way when you adjust the beam in.

    Install the original E21 beam bushes and ears on the car, have the E28 beam WITH the differential mounted ready to go in.

    First you need to mount up the trans axle. This will control the forward / backward adjustment of the beam. You can do what i did on the pic above, using a big washer and a bolt and nut and temporarely install the diff into the bottom of the car.

    And here are som VERY important info.

    Welding on the ears like this is WRONG!!!



    The problem here is that the beam will sit too low on the car, and the transaxle will have to work in a angle. In my case having a 2,5" exhaust the transaxle will not clear the exhaust. So it needs to be welded on roughly 10mm further up like shown on this pic:



    As you can see on the pic above the beam and the ear are not flush. So you will need to put in some sheet when welding the remaining hole shut.

    This will put the beam far enough up under the car so that the transaxle runs more straight into the diff, and will also reduce the risk of scraping as the E28 rear beam per definition just sits lower due to its somewhat larger size.

    This is a correct fitment of the beam on the ears. Please not the gap above the ear.



    Rear brakes

    Not all E28īs has a disc brake setup, and this upgrade is of course recomendable. Use E28 parts. I am not sure if the hand brake cable from the E21 will fit the E28 hand brake (small drum inside the disc) at this point. However the disc brake setup on the rear could be the perfect opportunity to set up a hydraulic hand brake system ?


    End notes

    Other things to consider is also the rear suspension. The E21 struts will no longer fit as the the e28 has some very large bolts holding the struts on the trailing arms. The original E21 bushings simply use a much smaller size bolt, so they wont fit. You can change the bushing in the strut, and this is reccomendable if you have some expensive suspension parts on the car allready. Otherwise E28 rear struts are a direct fit and can be used instead.
    Last edited by ruldal; 06-04-2015 at 11:41.


    Alot of people can say that they bought their dream car. Not alot of people can say they build it..

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    watty (09-02-2015)

  3. #2
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    Nice write up mate

  4. #3
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    Nice. Do you know anyone that has run this set up and put a few miles on it?

  5. #4
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    Thanks mate !

    Yes i am aware of at least two, one from Holland with a streetcar, and a guy from Germany that uses his for historical racing, the latter has run a couple of seasons with this setup, even though comparison is hard as he runs aluminium bushings and rose joints all over on both the rear and front ends.. One of my mates are just finishing off his (he is done with the setup, but are currently redoing the hubs for a 4 lug pattern), and i am in the process of doing it for my own as well.

    My mate has really gone and made a "bastardized" version using a E28 crossmember, E34 diff, E23 trailing arms, axles and rear brakes, E30 diff cover..

    I also know of a bunch of guys from Norway that does something similar using E30 parts spliced with the original E21 parts. That is all good, but there are one bad thing about the way they do it. They keep the small E21 axles, and that is really what this kind of conversion is all about, bigger axles..


    Alot of people can say that they bought their dream car. Not alot of people can say they build it..

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  7. #5

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    Hi Ruldal from a newbie !
    I know you posted this thread sometime ago and I must say its very well described, so thank you for that ! We (me and my two sons) fancy doing this E28 rear conversion to our E21 as we are heading for more horses upfront.
    What would be tremendous is if we could see the images, would you be able to share these electronically with us please ?

    Hoping you are still lurking on these forums !
    Thanks Roy.
    roysum@btinternet.com

  8. #6
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    Quote Originally Posted by roysum View Post
    Hi Ruldal from a newbie !
    I know you posted this thread sometime ago and I must say its very well described, so thank you for that ! We (me and my two sons) fancy doing this E28 rear conversion to our E21 as we are heading for more horses upfront.
    What would be tremendous is if we could see the images, would you be able to share these electronically with us please ?

    Hoping you are still lurking on these forums !
    Thanks Roy.
    roysum@btinternet.com
    Hi Roy. Sure thing mate, pics will be arriving in your mailbox tomorrow

    Sendt fra min SM-G950F med Tapatalk


    Alot of people can say that they bought their dream car. Not alot of people can say they build it..

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