DIY Installation: Rear Differential Bushing

4 Mar

You’re only as strong as your weakest link. To that end, there are many small changes you can make that will literally transform how your car performs.

One issue that plagues the Z and G is rear wheel hop. Some try to “cure” it with an aftermarket differential, only to find the problem magnified. The solution are some rather simple looking, but ultra effective bushing replacements. The Z and G have a rather conventional differential (aka the pumpkin, because of its shape…even though on these cars, it looks more like a squash) mounting system with 2 “ears” at the front and a single, large rear bushing. The front set of bushings are mounted into the pumpkin casing itself. The rear bushing, however is mounted in the subframe. All these bushings are liquid filled rubber, encased in an aluminum shell. OEM’s use this style because its reasonably stiff and strong, but able to dampen out noise and road imperfections. The whole rear differential assembly weighs about 90 lbs, so those bushings are under tremendous strain as the car squats, launches and turns. What many owners find is the rear bushing eventually starts to weep its liquid out, eliminating its effectiveness. The tell tale signs are a black stain on the rear subframe. The subject car here didn’t have that issue, but that does not make the result any less awesome. On this car, the front bushings had previously been replaced with the solid SPL units several years ago. The rear most bushing never was done due to time constraints at the time. But that’s what is being tackled here.

Step one involves using some PB Blast and getting under the car and soaking the bushings. This will cut through any surface rust that may have developed, and give the factory bushings some slickness to help in its removal. Step 1.2 starts with unbolting the mid pipe, and loosening the rear swaybar brackets. This lets the bar spin upside down, granting you more room to work. The pumpkin comes out without the bracket removal but you will appreciate the room when reinstalling it. Next, remove the 4 bolts that connect the rear driveshaft yoke to the pinion flange. Next up, unbolt the output shafts from the axles. The axles will dangle in place which is fine. Next step is drain the differential fluid via the side drain bolt. From there, unbolt the speed sensors at either side. Be careful! your speedometer and ABS use these, so unbolt em and tuck them up top. If you can reach it, use a pair of needlenose pliers and remove the breather hose at the top of the pumpkin. Next, you’ve got the 2 14mm bolts at the front of the pumpkin, and the rear nut that is “in” the bushing in the rear subframe. A tranny jack and a friend are very helpful here. 90 lbs is a lot and this isn’t something you want to drop!

Rear pumpkin with axles disconnected:

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Next up is the big rear cylindrical bushing. Some people stop the whole subframe and use the opportunity to also replace the bushings that mount the whole rear cradle to the chassis. For this job, we are leaving the subframe in place. There are several methods to remove the large factory bushing. What we chose to do is use is a traditional removal tool to push the bushing out. Others choose to drill through the factory rubber, then saw several slits through the casing to collapse the bushing. Both methods work, just depends on your preference and tool collection.

Removal Kit:

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The removal kit works like a plunger. You have a “pushing end” and a receiver. A bolt rides through the center and is secured with a nut at the other end. As you tighten the assembly, the stock bushing is pushed through its residence until it “falls” into the receptacle. Going slowly is key as is generous amounts of PB Blast. You must ensure torque is applied evenly to avoid doing any damage.

Stock bushing removed with the help of a bushing removal tool:

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Daylight!

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With the factory bushing removed its time to install the new one. We chose the SPL solid aluminum bushing to match the ones previously installed at the front. This is a solid chunk of billet goodness, and provides the strongest possible mount with maximum stiffness. Whiteline and Energy make urethane versions as well. If you go the solid route, a word of advise. A day before you tackle the install, put the factory bushings in the freezer. This will contract then ever so slightly, but will allow them to slide more easily into place. Leave them in the freezer till its time to install them.

To install the new bushing we used a simple mallet and tapped it in place. It’s actually quite easy.

New bushing installed:

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From there it’s a reversal of the previous pumpkin removal procedure. Make sure you get the pumpkin all the way squared up to the subframe otherwise you will never get those 2 front mounting bolts back in place. We found that by installing the rear bushing nut and tightening first, it “pulled” the pumpkin more into place allowing the front bolts to more easily thread in.

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Bolt the driveshaft up, then output shafts and you’re done! Torque specs can be found in this diagram:

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The results? Awesome! You will LOVE this mod. Even though the front bushings have been installed for several years the rear is the most transformative. The car bites down much harder now from a dig as well as in the turns. We noticed a slight increase in noise due to the fully solid mounts but its so faint it’s not even worth mentioning. Launch the car and that “hop-hop-hop-hook” sensation you used to feel is now just a squat and hook. Your axles will thank you……

With the affordability of these bushings, it’s on that list of “must have mods” for this car.

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3 Responses to “DIY Installation: Rear Differential Bushing”

  1. henry December 12, 2013 at 6:49 PM #

    curious where you got the bushing removal kit? i am doing this exact same procedure to my G (even down to the SPLs already being installed on the front end) and desperately need to get this job done. i dont really have the time to track down parts to make a DIY puller and would rather just buy a puller tool specific for this purpose. will any bushing puller kit work, or was this something specific for the G differential mount?

    • Z1 Performance December 12, 2013 at 7:32 PM #

      The SPL diff bushing comes with the die to press the bushing itself. Nothing car specific about the tool just a bushing press kit with receptacle cup. They usually come with a range of dies.

      • henry December 12, 2013 at 7:34 PM #

        awesome, that’s what I suspected but just wanted to make sure. thanks for the awesome walkthrough!!!! cant wait to get mine finished.

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