In a quest to figure out why my 2017 F54 S ALL4 has a floppy, cowl-shake feeling over some bumps on the highway, I discovered some terribly sloppy differential mounts. I’m pretty confident this is the issue - The differential bounces around like crazy over bumps.


The AWD F54 (plus F60, F39, F45, F48, etc) use 3 rubber bushings to mount the rear differential. Two large bushing mounted on a large aluminum beam at the rear of the car, plus one tiny bushing on the front right side of the differential.







Oddly the bushings on the rear beam don’t appear to be available separately. The R60 Countryman uses a similar set and they are available. More on that later…

The front bushing is part number 33366852897.

So here’s what I discovered with my two rear bushings. They’re not terrible, but not great.

They look like this:

And have play like this:

Not massive, but this is just laying on my back and awkwardly pushing up with one hand.

And another with the bushing out of the car. You can see the massive voids in rubber design. To be fair though, this example is not accurate for the loads this bushing sees while in use.

Even worse is the small forward differential mount.



With a massive amount of slop, just pushing with a few fingers upwards on the differential. This is accurate for the movement this bushing sees while in use.

 

To remedy this, I unfortunately haven’t found any aftermarket polyurethane mounts or stiffeners, so this means I’ll need to do some custom reinforcing.

For the front bushing, I’ll be using a new factory MINI mount, filled with Smooth On PMC-770 liquid urethane rubber mix. A 70 durometer two-part urethane system. I’ve used the PMC-780 (80 durometer) on bushing with much success in the last. More on this in a later post.

As for the rear mounts, my first idea is to look at the R60 Mount kit from Powerflex. The factory R60 mounts look very similar to, but not exactly the same as, the F54/60 mounts. They also look very similar in size. I have a couple messages in to Powerflex and other companies to try to get the dimensions of the R60 mounts, hoping they’ll swap right in. Here is the Powerflex PFR5-1825 kit for the R60.


If those don’t work, I’ll be filling the factory mounts with Smooth-On PMC-770 too.

More to come….

 

All that movement is designed into those bushings to combat NVH (Noise/Vibration/Harshness), but certainly doesn't do much for looks when you see how much movement you end up getting.

Back in my SVT Contour days, wheel hop on wet surfaces was a huge issue due to the engine mounts allowing too much movement. They were similar to those suspension bushings with a lot of air/void space and soft rubber. The poly inserts you could buy would help tighten things up, but you still had a lot of 'squishy' rubber. The solution was to remove a large portion of the rubber and replace with some available urethane, specifically 3M Windo-Weld (https://www.3m.com/3M/en_US/p/d/b40067015/). I did this on my engine mounts and it worked fantastically, where you really, really had to try to induce wheel hop. The process is slow, where it took a full week or so to completely fill in the bushing. Some folks would simply fill the entire cavity in one shot, let it sit for 24 hours and call it good...they didn't end up with good results as the urethane in the middle of the 2" wide bushing wouldn't cure in 24 hours. In fact, it takes ~2 weeks to cure fully through. I learned that doing a 1/2" thick layer every 24 hours sped up the overall process and gave you a very solid urethane fill. Depending on your car's use, this may not be a viable option, but something to consider.

 

I received the dimension for the R60 Countryman rear bushings from Powerflex. They are very similar to the measurements I took on the F54. I think they might work on the F54, but I had Smooth-On PMC-770 ready to go so I filled the rear factory mounts with it.

I was going to fill a new forward mount but it’s currently backordered. That be another day.




After removing the rear differential carrier (easy, just a 16mm socket) I thoroughly cleaned the rubber mount then started sealing one end of it up to pour the urethane rubber mix in. Mostly just hot glue and a cereal box 😄.




Mix the product as directed and pour into the voids. Pot life on this is 30 minutes, but after 15-20 minutes it’s getting pretty thick. Here’s the mount after about 12 hours of cure time and the hot glue molds removed. Not pretty, but 100% functional.




Then I did a little trimming and cleaning up the new urethane and reinstalled the rear beam.




I’ll wait until I modify and install a new front bushing before doing a review. Access to the front bushing is extremely tight, and the bushing is on its side, so it would be difficult to use the liquid urethane method while the bushing is installed.

^ I actually did consider 3M Windo Weld for the front bushing before I bought the PMC-770. I have used Windo Weld in the past too. I’ve also used PMC-770 and 780 in bushings for many years with great success so I wanted to stick with it.


I do see on BMW vehicles with this same chassis (X1, etc), there’s a large 2.5kg weight bolted to the center of the differential beam for vibration dampening. My F54 only had two little weighs next to the mounts, even though the part is listed for my car on RealOEM. I hate adding weight to the car, but I’d be curious how much that BMW weight would change things.

Part 33366868778

 

Looks good, yet another option for filling the bushings. It seems that it may be a bit softer than the Windo Weld, but anything filling the air gaps would certainly help. Would be good to see how it holds up over some time. I don't have any photo reports on the Windo Weld of the engine mounts, since I sold that car ~4 years post modification, but I never noticed anything hanging/falling out of them during oil changes in the years after.

 

I made some progress on the forward differential mount. I sealed one end of a new bushing with some floor protection film. It has a slightly sticky back with a light, removable adhesive. It’s essentially vinyl wrap film, which should also
work great if you have scraps around.








Fill it with Smooth-On PMC-770, let cure, then remove the the film.







I’m super happy with how it turned out. Next challenge will be installing it.

 

Got the modified forward bushing installed. Not terribly hard, but you do need to drop the exhaust, unbolt the driveshaft from the differential (3 bolts), and lower the differential about 12”.

My homemade bushing press tool had a hard time getting the bushing out (my fault - didn’t get hardened threaded rod), but the new one went in fine coated with anti seize. The bushing shell is aluminum, so cutting the rubber out and using a hacksaw to cut the old shell worked well. Then hammer the old bushing shell out.

I’ll update this once I get a chance to drive it more.