This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the most common parts to deteriorate on the 911 is the engine and transmission mounts. Because of the design of the 911 drivetrain, the mounts don't wear out as much as they do on some other Porsches, but they still require replacing after many years. The rubber that is contained within the mounts becomes old and brittle, and doesn't perform a good job of isolating the drivetrain from the rest of the chassis.
Old, worn out motor and transmission mounts can cause shifting problems because the drivetrain is no longer firmly held in its position. One sign of this failure mode is the gearshift knob jerking backwards under hard acceleration or difficulty selecting gears during cornering. A visible sign that the motor mounts need replacing is the appearance of cracks in the rubber of the mounts. The rubber will deteriorate over the years and need to be replaced, even if the car has relatively few miles on it.
The first step in replacing the motor mounts is to place a jack under the engine and support it. Make sure that you use a block of wood or some newspaper to protect the bottom of your engine case from damage. See Project 1 for more details. Once the engine is supported, you can begin removing one of the bolts that fasten and hold the motor mount bar to the mounts. The mounts themselves are located in the engine compartment, along the back shelf (see the photo in Pelican Technical Article: Engine Removal, Engine Removal). Remove the long bolt from the center of one of the motor mounts. On the later cars, these bolts are threaded into the motor mount bar itself: there is no nut on the other end. These bolts are often rusted in place and should be lubricated generously with WD-40 from the bottom. A sticking bolt can actually bend the mounting cross-bar when you are trying to remove it.
It is advisable to replace each motor mount, one at a time. Otherwise, the weight of the engine may cause it to shift, and it may become misaligned. Also, the motor may drop down slightly from its normal position. Replace the mount on one side and then replace the one on the opposite side.
Once the center motor mount bolt is removed, then you can simply remove the entire motor mount by removing the two smaller bolts that attach it to the chassis. These bolts have corresponding nuts that must be kept from turning when they are being removed. Access to these nuts is gained by reaching down underneath the shelf. Once you have the motor mount removed, then simply install the new one in the same place. Tighten up all the nuts, and perform the same procedure for the opposite side.
The transmission mounts are very similar to the motor mounts, except for the early cars. Unfortunately, the early cars have a once piece mount and support bar design that is no longer available. The rubber transmission mounts are embedded into the two foot wide support bar that mounts the entire transmission to the chassis. Sometimes, good used mounts can be found, but they are getting scarcer every day. To replace this one-piece mount, support the transmission with the jack, and unbolt the mount from the chassis and the transmission. Depending upon the configuration of your car, you may have to remove the rear sway bar in order to remove the transmission mount.
The later style 911s (1972-on) were equipped with a transmission mount bar that incorporated replaceable mounts. These mounts are very similar to the engine mounts, and should be replaced in a similar manner. Make sure that the transmission is well supported with a jack before you remove either of the transmission mounts. As with the engine mounts, work on one side, and complete the installation prior to removing the mount on the other side.
When you are finished, you should feel an improvement in the shifting of your car, and the drivetrain vibration should feel a little tighter and less sloppy. If you are a hardcore racer who wants a really stiff drivetrain, then you should consider upgrading to solid mounts. Keep in mind however, that that solid mounts will make the ride a lot less comfortable and much more noisier.
The transmission mounts can easily be seen from underneath the car. Shown here are the mounts on a later-style 911, which are replaceable. On the early cars, these mounts are embedded into the steel transmission mount bar and cannot be replaced. Used transmission mount bars with good rubber mounts can still be found, but they are getting increasingly difficult to locate.