E3/E9 Rear Differential Metal Stress Fracture...

[B10er]

Hi,

I own a 73 Bavaria. Recently I noticed a small hairline crack (3 inches or so) on the centre floor of my trunk.

My understanding is that this is a stress fracture caused an old rubber differential mount, and is a common design flaw in these cars. I'm not all that familiar with e9s, but I've seen neglected Bavarias/e3 where the entire trunk is split by a crack of over a foot long.

My understanding is that the repair is (1) the removal of the rear diff., (2)weld up the hiarline crack, and (3) weld in a reinforcement plate in the effected area for many years of future happy motoring.

Has anyone had experience with this/has comments to offer/can point me in the direction of information about this? If all goes well, she'll be going under the knife this month...

Thanks!

Chris

[Gordon]

The problem you described is much more common on E12's than E3's. It sounds like you found the crack before it became a huge problem. Check out this article on E12 differential mount repair from the firstfives.org technical FAQ forum. I'm going to forward your post to the E3 email list and see what they have to say about your problem.

http://www.firstfives.org/faq/diffmount/bulletin.html

G-Man

[c.d.iesel]

sounds just like the E46 rear floor failure -

control arm mount floor ripping apart ? not pretty .

Sad BMW designs.

[JohnS]

I seem to remember a picture on Peter Sliskovich's website:

www.coupeking.com

Where he shows how he reinforces the diff. mount area on the E9 coupes.

[BarryA]

with the E12 {early} and E28 {late}) - the rear mount on my wife's car suffered complete failure last year (rear of the diff was almost dragging on the ground - fortunately, the failure happened a block from home).

Her 633 is a gently adult driven, 2 owner / CA central valley from new, rust & damage free auto trans car, so it definately was NOT due to abuse.

[Gordon]

Early E24's share the same pan structure as E12, then the E24 pan shifted in 82 to a E28 base pan which had significant structural reinforcement compared to the early E24 pan.

The consensus in the E3 community is the diff mount problems are almost non existant among E3 owners. They were thinking that if you drove your E3 like your B10, you may have problems with your diff mount.

Here's the E9 reinforcement on Peter's webpage

http://www.coupeking.com/product_id-410.html

I've heard that E36 M3's had a lot of problems with diff mounts as well. That's a lot of torque going through the drivetrain on a E36 M3 and the diff mount has to asorb a lot of it.

G-Man

[B10er]

Thanks to all the replies - lots of good info!

G-Man - thanks very much for the check-up. I thought I'd attach a picture - if possible to show it to some other e3 guys, I'd really appreciate it. I've seen two Bavs recently with huge cracks in the same place.

[B10er]

As for driving it, I wish I'd have had enough opportunity to drive it nuts hard, but between sorting and winter storage, I don't think I've drivin it more than 2000kms in a couple of years, and 1500 of that was on one weekend!

Just for fun, here's another pic of the car in question: