M coupe Part 2: Differential Refresh

The M coupe I bought did have a small rear diff leak, which was known at the time of purchase. I did not know at the time of purchase that it had a 3.23 LSD instead of the stock 3.15 LSD. Had I known that I may have done things a little differently. Was all this work wasted? Maybe. I'll explain at the end...

The goal was to restore a 3.46 Torsen diff I had from a 3.0i Z3 parts car and prep it to install in the M coupe. Then I started working on it and thought, why not do a mild rebuild on it? I didn't go so far as replacing bearings - but I replaced the seals and clips. In retrospect I probably should have just done a full rebuild, but I hadn't gone this deep into a diff previously, and didn't want to bite off more than I can chew... anyways, this is what I started with. I cleaned it well, using a combination of a few things, pressure washer, brake cleaner, some mild abrasives. It wasn't too bad, my environment is pretty gentle on metal - no major corrosion to speak of.

The input flange before and mid-sanding. I was initially sanding this by hand, then switched to some abrasive rotary wheels to get a nicer end finish.

Sprayed clean and dried again, to get any residual bits from sanding off the surface before painting.

I went ahead and installed studs in the diff as well, to ease installation. I only install these on the forward most mounts - the reason for this is two-fold. The main reason is that the goal of the studs is to ease installation when removing from the subframe. I've found that having studs in the rear mounts can make it very hard to get a nut up and over the stud, as access to the top of the subframe in this area is very limited. So the studs in the front and the factory bolts in the rear has proven the easiest way to install them for me. The second reason, which is really more a convenient side-effect, is that the kit includes four studs and by only using two per diff I can install it on two diffs per kit instead of just one. Just a little red loctite on there and they are good to go.

Next was to mask and paint. I wasn't sure what paint was best, I ended up using a Rustoleum "Metallic" which I had on hand and while I love the color and finish, I am not entirely convinced of it's durability yet. Time will tell. With the studs, it will be easy to uninstall and repaint in the future if it starts to look bad.

I put some bolts in the holes to protect the inside bore on the flanges, left the old seals in for this step and a rag underneath to protect the inside bore. After painting I then used a razor to scrape off the paint that made it past the bolt heads. Did the same process with the razor on the output flange mating surface. Worked very well.

This is the diff after painting and finishing the buffing/polishing. Note all the bolts are not installed - just enough to hold the flanges on for now. I have the original bolts being sent off to be zinc plated.

I then moved onto the output shafts. I was originally planning on cleaning up the outer part which was most visible. I ended up cleaning the entire shaft. Note also that the shaft on the right was also cleaner than the ones I started with - I found this one after I was done (and after I forgot a proper before/after picture, again) to take a comparison pic for. The ones I started with were basically completely black. I have a timelapse of the process I will work on uploading soon.

With the shiny M output flanges, a shiny diff!

After all this though, I am second guessing whether I want to install the 3.46 in my M coupe. It currently has a 5 speed transmission and I do a lot of highway driving - so managing highway rpm is critical for me. I'm already turning over quite a few rpm with the 3.23, I will have to run some numbers to see where the 3.46 will put me. I could always shelve it, repeat this process for a spare 3.23 I have (and do a FULL rebuild, clutches, bearings, etc maybe?) and install the 3.46 if I go 6 speed down the road, or just keep around, I have a few Z's after all. I definitely have some thinking to do

Up next... a pointless endeavor


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