The complete strip-down of my E36 M3 chassis is nearing completion.  With my recent garage clean-up I can actually get to the areas I’ve long been neglecting.  One of the last remaining “large” jobs was removal of the rear subframe.  I needed to do this for a number of reasons, chiefly that the whole suspension needed to be restored and improved.  Secondary to that main effort was that it would making complete removal of the wiring harness easier, or even just possible if I wanted it completely intact.  The rear wheel speed sensors and differential speed sensor wiring runs through a large rubber grommet above the differential then down and out to the various sensors.

Having the car already up on jack stands is a must, how convenient that mine has been hibernating in such a position for many months.  Now the process of dropping the subframe is pretty straight forward, the whole assembly is only secured by two nuts and two bolts to the uni-body.  There are, however, several other items that need to be disconnected prior to complete removal of the unit.  The parking brake cables must be pulled out.  Again, a very simple job given that the hand-brake assembly was completely removed a while back.  The cables slide out of their steel tubes with a little tug.  Then the brake lines themselves that run to the two rear calipers.  Since I wasn’t going to reuse the 200k mile rubber lines, a pair of heavy-duty wire cutters made simple work of those.  The speed sensors just unplug from their sockets and are out of the way.

I wanted to drop the rear subframe and suspension as one complete unit, which would just make life easier.  To do so you need to unbolt the trailing arms, which are secured with three bolts in pockets just ahead of the rear wheels.  An impact wrench pulled all but one out, which I had to persuade with a little PB Blaster and a breaker bar.  Both trailing arms will drop down once unbolted, giving you a good view of the condition of these pockets, a known weak point in the chassis.  Lo and behold, my chassis did not escape this troublesome area.

Rear trailing arm mount damageYou can clearly see in the photo above how one of the threaded holes has pulled away from the sheet metal in the body.  This is the exact reason several aftermarket companies provide weld-in reinforcement plates, of which I have sitting in my parts storage room (aka home gym.)  So far, so not so good.

With the trailing arms disconnected I could move on to the last items that need unbolted before getting to work on the rear subframe itself; the shocks.  Again, a simple process.  You can remove the two nuts at the top of the shock towers that hold the shocks to the body or the bolt that connects the shock to the trailing arm, or just remove both and have the shocks completely out of the way.  I just used the small Ridgid electric impact to zip off the two nuts at the top of each shock mount, just one requiring a bit of PB Blaster to get moving.  With the trailing arms and shocks disconnected the coil springs should just fall out or at least be easy to remove by hand.  Now everything is disconnected and ready to drop the entire assembly.  But how does one get it down?  If you’ve ever lifted a BMW 188mm differential, you’ll understand just how much of a pain this can become.  These beasts are heavy, around 90lbs just for the diff alone, much more considering I have axles, trailing arms, upper arms, camber arms, rotors, calipers, and the subframe itself to contend with.

I had considered just doing it the way I had in the past, with a trusty old floor jack.  The main issue with this is trying to balance the whole assembly on a narrow jack pad, that’s never easy (even though I managed to manhandle a 6-speed up with one.)  Rather than risk a hernia, I hit up the local Harbor Freight to see what crappy Chinese knock-offs could handle the job.  Let it be known that I absolutely hate the tools that come from this place, but for a one (or two) time use items like a transmission jack, it’s hard to justify the price of a name-brand item.  I ended up picking up a cheap scissor-style transmission jack.  I figured it would provide the best method of removing and installing a subframe, as it lifts straight up and down.  You want this as the subframe slides on and off two long studs threaded into the body.

Harbor Freight Trans Jack Scissor Jack

In the photos above you can see the jack pressed up against the bottom of the differential.  It comes with a ratchet strap to secure a transmission, which was mostly useless in this situation.  I used a thick piece of rubber floor matting under the rear of the differential to help balance the assembly, as well as adding another ratchet strap around the input flange.  I raised the jack using a cordless impact (the jack has a 1/2″ drive fitting) to where it was pressed tightly against the bottom.  From there I used the same impact driver to remove the two bolts and two nuts holding the subframe to the body, again one require the persuasion of a breaker bar.  Now that everything was free, I just lowered the whole unit down slowly, piece of cake.

Lowering Subframe

With the jack dropped all the way down, I then just wheeled the whole lot out from under the car.  Then I could lay eyes on the rest of the underside of the car, as well as more closely inspect the subframe and associated components.  That’s when I found even more surprises.

Subframe Damage 2 Tire Well Damage 2

Tire Well Damage 1 Diff Cover Damage 2

Diff Cover Damage 1 Subframe Damage 1

No that’s not good news at all.  The photos above show that at one point this car had some serious rear end damage.  The subframe attachments that hold the twin-ear differential cover are badly twisted.  The cast aluminum differential cover itself had been cracked and welded back together on its right ear.  The spare tire wheel well has a nice dent, probably resulting from the entire subframe being pushed into it.  The saving grace is that the frame rails themselves, which are integral to the uni-body, don’t appear damaged or repaired in any way.  Bottomline: a new subframe is in order, as well as some repair work for the tire well, and I wouldn’t feel comfortable racing with a welded cast cover still in place.

Building a race car is an adventure right?  Well at least that’s how I’m looking at it!


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