Replacing Rear Shock Mounts on Your BMW Wayne R. Dempsey
Difficulty Level: 4 Difficulty scale: Adding air to your tires is level one Rebuilding a BMW Motor is level ten
This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
The 3 Series rear shock mounts (RSMs) are weak. These mounts hold the top of the rear shocks to the chassis. Repeated wear and tear on these mounts causes them to crack and break after many years of use. The worn-out shock mounts result in a clunking noise that emanates from the rear suspension while you’re driving. Left unchecked and unfixed, the broken mounts can inflict permanent damage on the chassis by tearing the sheet metal in the rear wheelwells.
Begin by opening the trunk and removing everything from it—including the carpeting. If you have an E36, you’ll also have to remove the rear speakers to gain access to the rear shock mount (see Photo 1). If you have a convertible, the rear shock mounts are located in the convertible top compartment behind the rear seat. Carefully pull back the molded carpeting in the trunk and remove whatever carpet fasteners hold the carpet down. Then remove the two retaining nuts that hold the rear shock mount to the chassis.
At this time, also disconnect the rear shock where it attaches to the rear trailing arm (see Project 61). Remove the entire assembly from the car disassemble it on your workbench (see Photo 3).
Around the time that the Z3 was designed, BMW realized that this rear shock mount design placed a lot of stress on the sheet metal surrounding the rear shock towers. As a result, BMW engineers have included a top-mounted support bracket that sandwiches the rear shock mount and distributes the load better (part number 51-71-8-413-359, about $15). BMW also redesigned the mount for the E46 3 Series cars, making sure the mount was backward compatible with the earlier 3 Series cars. This upgraded mount is used on the E46 M3 and E46 convertible cars, and is the mount to use if you swap out your rear shock mounts for replacement stock units (part number 33-52-6-754-096).
If you wish to go a step further in performance, install an aftermarket rear shock mount kit. I prefer the aluminum billet kit manufactured by Ground Control and available from PelicanParts.com for about $150. This kit replaces the rubber inner bushing with a polyurethane bushing that is much stiffer and far more secure than the stock mount. In addition, the aftermarket kit contains a beefy support plate that reinforces the sheet metal at its weak point. Installing the aftermarket kit is very similar to installing the stock mounts.
Reuse the plastic dust cover and the two concave washers from your old shock. Don’t reuse the rubber bumper, as these are typically near disintegration by the time most people replace the mounts. Use new self-locking nuts on top of the shock, and affix the rear shock mount to the chassis. Finally, use a new gasket to seal the base of the shock mount to the chassis. Reinstall the shock mount and replace the rubber boot on top if it was there when you removed the old mount. Reattach the shock at the bottom, reinstall any speakers or carpet removed, and you’re finished.
If you find your chassis sheet metal torn around the mounts, there is a BMW factory retrofit part that can be welded into place to repair the metal (part number 41-14-8-169-027 for the left side and 41-14-8-169-028 for the right side). Count on spending about $100 or so for a good welder to fit and weld these plates into the rear of your trunk.
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Figure 1
To access the rear shock mounts, remove everything in your trunk. On E30 cars, you only need to pull back the carpet, and the mounts should be easily accessible underneath. On E36 cars, the rear speakers interfere with access to the mounts (yellow arrow). From the back seat, pry off the speaker grille and remove the two screws that attach the speakers to the chassis (green arrows, lower left). The speakers should drop into the recesses of the trunk (lower right). No need to unplug them—the wires should be long enough to simply place them aside.
Figure 2
Here’s a photo of what the rear shock mounts in the rear of an E30. On some cars, there may also be a factory rubber boot installed (inset photo). The red arrow points to the actual rear shock mount (RSM), and the blue arrow points to the very tip of the rod that runs the length of the rear shock.
Figure 3
With the rear shock assembly removed from the car, disassemble it by removing the top retaining nut from the top of the shock. Hold the shock tip with a pair of locking pliers or a similar tool. Closer examination of this rear shock mount upon disassembly reveals a large crack (red arrow).
Figure 4
On the left is the aftermarket rear shock mount kit from Ground Control. If you’re planning on some spirited driving, this kit is just what you need. On the right is a stock mount with the Z3 reinforcement plate, a less-expensive alternative to the aftermarket kit.
Figure 5
This photo shows the proper assembly order for the rear shock mounts. Red arrow: shock absorber dust cover. Green arrow: rubber/foam bumper stop. Blue arrow: lower cup washer (cup faces the rubber/foam bumper). Purple arrow: rear shock mount (stock unit shown). Yellow arrow: shock mount/body gasket. Orange arrow: upper cup washer (cup faces upward toward the top). White arrow: shock retaining nut.
Figure 6
Here’s a photo of the finished product. Secure the top of the shock with a set of locking pliers so you can tighten and affix the top locking nut (inset). Only use new locking hardware—don’t reuse the old hardware here. Add the Z3 reinforcement plates to the rear shock mounts (green arrow). They are very cheap insurance against tearing your sheet metal.
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