My gearbox mounts were collapsed, so I decided to replace them. Not much info on this work so I did a write-up with pic's. I'll send the rest of the pic's and of the new mountssoon.
Gearbox mounts replacement on an 1989 928 S4 Rhd 141,000 miles.
1) To gain access to the gearbox mounts the car must be raised as high as possible, preferably on a lift or axle stands if that's what you have. You will be spending quite sometime under the rear of the car under the gearbox/diff/exhaust etc, so Do NOT raise the car on jacks alone as that is super dangerous for this type of work.
2) Remove the rear wheels. This allows more light to access the gearbox area helping you to see things clearer and removes items you may frequently bang your head on!
3) Find the gearbox mounts. They're located at the back of the gearbox on the side just forward (towards the front of the car) of the half-shafts. Find the inner CV boots attached to the tranny and then look towards the front of the car and you'll see a square shaped rubber mount just slightly above the CV boot. There's a large 19mm bolt at the top holding the mount to the gearbox and one at the bottom securing the mount firmly to the weissach axle.
4) Inspect the mounts. Depending on mileage, you'll probably find that they've compressed a lot or even fully, so that the triangle part at the top is compressed a lot. See pic's of mine which were collapsed. To further verify knackered mounts, look at the gap between the gearbox and rear cross-member. The gearbox should be nearly 2 inches away from the cross-member. Mine was only a quarter of an inch!
5) The bolts look easy to access/remove; don't be fooled; the top ones can be a bitch! They've been there 30 years so require a lot of force to loosen. Start on the passenger side lower bolt as this is easiest. Easy access for the required 19mm socket with ratchet, breaker bar or long bar like I used. TOP TIP: Before cracking the bolts off, use a needle file or paint to mark the bolt head and cross-member in a way that shows you where the bolt was tightened too before removal. I scratch an arrow into the bolt head so that when I wind it back in I know where the bolt was when torqued down. Hence, I will not over-torque the bolt when reinstalling so avoiding stripping the threads. Just get the lower bolt cracked off but DON'T remove it. YOU MUST ensure you can loosen ALL FOUR mounting bolts before removing either of the mounts and support the weight of the gearbox with a jack first before removing the mounts.
6) Move on to the upper 19mm bolt. Access for a socket is limited. I used a 3/8 flexible extension on a short 19mm socket with extensions to a ratchet as shown on the pic's. To get enough leverage to crack the bolt loose I also attached my long, hollow steel bar to the ratchet. Be careful to keep forward pressure on the socket on the 19mm bolt otherwise you'll easily slip off these. All being well, the bolt should loosen with perseverance. If not, take a break and come back later and persevere. It took me 4 attempts on each mount to break the top bolt loose, each time working out what was the best way to access the bolt with removing stuff (exhaust/half-shafts etc). For other 928 peeps, I've only shown the method that worked! Again, only crack the bolt loose and don't remove it yet.
7) Move over to the driver's side. Start with the bottom bolt as it's easiest. The handbrake cable obscures the bottom bolt so it has to be loosened and pulled down and away. Loosen the handbrake cable at the central connector after taking your handbrake off (see pic). With the cable out of the way, mark the bolt and crack it off. A 3/8 19mm socket is what you need here as the access is limited. Crack it loose and leave in place.
8) Now for the driver's side top bolt; a real bitch! Mine was not only difficult to access but VERY tight. Again, I tried various ways to access the bolt with a socket, spanner etc. I've shown the best method that loosened the bolt for me. EVERY other method allowed the socket to slip off and mash my knuckles in the process! You can access this bolt from the wheel-well, but I couldn't get on it straight so the socket kept slipping off. My final and best method is shown; see pic's. Again, the only way this bitch moved was with a long bar attached to the ratchet for leverage. Ah, the relief when this bitch cracked loose! I spent 2hrs on this bolt alone!
9) Now support the weight of the gearbox. I used a floor jack with my bar resting on it and a 928 jacking pad to the diff. Don't go crazy jacking the gearbox up real high. Just support the weight for now.
10) Now you're sure BOTH mounts can be removed as the 4 bolts are loose you can proceed to remove and replace the mounts. I did one at a time as I always tread with caution. As the new mounts will be longer than the old compressed ones you'll probably need to slightly jack the gearbox up to align the top holes for install. Use LOTS of anti-seize compound on those bolts! Don't over tighten them as stories of stripping the top ones have been heard. It's difficult to get a torque wrench in to the top bolt so this is where your marks on the bolt come in handy.
11) Use the workshop manual to get your shim settings correct. Or just reinstall the shims that came out with the old mounts if you find them with your old mounts. Some 928s did not have them, other 928s do have them.
12) With the new mounts install/bolted down, you can drop your jack down. Replace your handbrake cable middle connection and your rear wheels. Go drive the car and notice any difference. General feedback is the suspensions seems better, less vibrations/bottoming out and smoother gear changes.
13) Original mounts are apparently no longer available from Porsche. I got mine from Porsche in the UK (£270 for the pair) who informed they were the last set in stock with them. US suppliers list them (Pelican Parts/928 International still have them but they're pricy). Design911 here in the UK still list them, but again they're expensive.
Hope this helps other 928 owners with this work. Now on to the dreaded engine mounts!
I forgot these helpful links for the gearbox mount work Theo:
Hi Theo, here's the further info/pic's on gearbox mounts renewal.
REMOVING THE OLD MOUNTS. The passenger side is straight forward. With the 2 bolts removed, jack the gearbox half an inch and then the mount will slide out backwards (towards the engine direction). See pic's. With the mount out, now is a good time to put some anti-seize on the top bolt and wind it back in as access is possible with your hand now the mount is removed. Doing this now means winding the bolt back in with the new mount on will be possible by hand until you have to torque it down.
The driver's side is more difficult as the gearbox ATF reservoir obstructs access to the rear. With the engine jacked half an inch you can wriggle the mount free with your hand. Now rotate the mount so the top bolt hole is facing downwards. Now push/pull the mount out passed the weissach axle crossmember towards the wheel hub/disc assemble. I t will probably require a fair bit of wrestling to get out as space is very limited. Again, a good time to grease and wind in your top bolt so reinstalling by hand will be easier with the new mount. I also greased the areas where the mount touches metal at the back, top and bottom.
With the new mounts on, you can reassemble everything and feel good on another 928 job done!
Hi Theo, here's the last of the pic's installing the new mounts.
A few tips:
1) Put some grease on where the metal part of the mounts rubs on the cross-member/gearbox. These mounts will be under a lot of tension so the grease will reduce friction/wear. Keep the weight of the gearbox supported by the jack as you install the top bolts into the new mounts. Get them in by hand first (why it's good to lube them and wind them in by hand and then remove them first, while there is no mount in place; very easy to wind in with them new mount in place). Don't torque them up yet.
2) Install the lower bolts and tighten them up a bit but not fully. Now torque your top bolts up. DON'T over-tighten them as the THREADS CAN STRIP! This is where the marks I made on the bolts help you know where the bolt was tightened too before removal. Sane thing I did on the lower bolts; see pic's.
The difference? Gear change smoother. Rear ride height sitting about half-an inch higher. No bottoming out/cluncking from the rear end. Rear end feels lighter on the road and through corners instead of heavy and wallowy. I can feel how heavy my front end is now as the engine mounts are collapsed, so I'll renew these next.
The work took me from start to finish 6hrs.