Project Summary:

This installation will upgrade the rear open differential on an AWD Impreza, Forester or Legacy to a rear limited slip differential (LSD). The only Impreza's with rear LSD are all 2000-2001 2.5RS and 2002 WRX models.

Here is a table of various AWD Subarus with their respective drive ratios:

Other North American Subaru models with LSD, either as standard equipment or available as an option:

• 2000-2002 Forester S (All Weather Package)
• 2003 Forester XS
• 2000 Legacy GT, Outback
• 2001-2003 Legacy (all)
• 2003 Baja

This documentation is based on the install on my 1997 Subaru Impreza Outback Sport, which has a 3.900 final drive. Make certain that you find out what final drive you have on your car, and only replace with a LSD unit that has the same final drive ratio. If you don't match the ratios, you face the possibility of overheating your center differential and causing it to fail.

This does not discuss swapping the LSD "pumpkin", as a shimming kit and special procedures are required to fit the pumpkin into your differential case. I believe that this is something beyond the scope of a home garage mechanic. (Or in my case, an apartment complex parking lot wannabe-mechanic.)

Note: these instructions are intended for someone with reasonable automotive repair experience and knowledge. Many details have been omitted, as they can be found in other literature such as Haynes' excellent Subaru Legacy Automotive Repair Manual (1990 through 1998). Missing details include: suspension assembly/disassembly, removal/installation of the LSD unit using a transmission jack, and rear differential protector details. However, I have included pictures wherever possible to aid presentation and to provide helpful hints.

Materials Required:

1 complete and functional rear LSD assembly from any 1993-2001 Impreza, Forester, or Legacy with LSD, consisting of:

Rear LSD unit with matching final drive gear ratio
Axles (optional, depending on model)
1/4" x 2" spring pins (optional, depending on axle)

Anti-sieze.

Gear oil, SAE 75W-90 or equivalent.

Shop rags

Note: I obtained my rear LSD assembly from a junked Legacy Turbo AWD from F&S Auto Parts, an established Subaru used parts specialist in Roxbury, CT, via mail order.

Tools Required:

14mm, 17mm, 19mm socket (ratchet) wrenches

27mm axle flange socket wrench (optional)

32mm hub socket wrench

12" or longer breaker bar

4" and 6" socket wrench extensions (optional)

12mm, 14mm, 17mm, 19mm open-ended wrenches

Large prybar or flat screwdriver

2-ton floor jack(s) or equivalent

2-ton jackstand(s) or equivalent

Let's Start

Before jacking up the car, it is advisable to loosen the axle nuts with a 32mm socket and breaker bar. Do this with the tires on the ground and the handbrake fully applied - it will give you a much bigger mechanical advantage than when the car is up on jackstands. Jack up the rear of the car. Use a block of wood to prevent slippage. Rest the car on two jack stands. Remove the wheels. If you forgot to loosen the axle nuts and have a helper, have him/her stand on the brake pedal while you loosen/remove the nut. Remove the rear lateral link-to-knuckle nut and bolt. This is the very long bolt that connects the two lateral links to the knuckle. Be prepared with some lubricating spray and a lot of wrenching to pull the bolt out. If applicable, use a large pry bar to pop the axle differential offet joint (DOJ) out of the differential. Use one of the bearing retainer bolts to leverage against. Pull the knuckle outwards to provide room to remove the axle.

Remove the four 12mm bolts and nuts securing the driveshaft to the differential input flange. Remove the 17mm nuts securing the differential to the rear crossmember and the differential cage. Drop the differential using a transmission jack or shop jack. Be extremely careful, as the differential is very heavy! Use a jackstand to hold the differential cage up.

If you're lucky, the LSD unit will be a drop-in replacement for the open unit in your car. For example, an LSD from a 2000 2.5RS will swap directly into a 1998 2.5RS without any other changes. Unfortunately, the LSD from a 1991 Legacy Turbo had three issues with my car: 1. The rear crossmember studs on the rear cover were too short. 2. The LSD has "outies" instead of "innies" for the axle stubs. 3. The input flange is a different size. Meanwhile, here is a view of the differential featuring an internal sealed viscous unit.

The differential crossmember bushing in my car is much longer than the one on the Legacy Turbo, so I need to have the longer studs. I was able to remove the studs from the LSD rear cover using a Vise Grip wrench, but the studs on the open differential rear cover would not budge. I gave up and decided to swap the covers instead. Torque the cover bolts to 22 ft-lbs. Now would be a good time to replace the gear oil. Rallispec recommends Mobil 1 Synthetic gear oil. Others prefer Redline Synthetic. I used Amsoil. Torque the filler plugs to 32 ft-lbs.

If your existing axles won't work with the LSD, you will have to get matching axles. See previous picture for differences between the "innie" and "outie" versions. And here is a picture showing the axles themselves. The top one is from my 1997 Outback Sport, while the bottom one is from a 1993 Legacy Turbo. I had to use two 1/4" x 2" spring pins to secure the axles to the LSD output shafts. Since all 1993-2001 Subarus have the same track, there are no issues with axle lengths.

The input flange on the 1991 LSD was slightly smaller than on my open differential, and used a different bolt pattern. I needed to swap the flanges to avoid buying a matching propeller shaft also. Subaru's service manual specifies using a Special Tool to hold the flange in order to remove the 27mm flange nut. The torque spec for the nut is 137 ft-lbs, so I would require a lot of leverage to break it loose. However, I do not have the Special Tool, nor do I have access to air tools. Fortunately, a colleague at work told me about a $20 impact wrench at HarborFreight.com that might do the job, although he wasn't sure how it worked. It is marketed as a roadside repair tool to untorque wheel lug nuts. I decided that a $20 impact wrench was a better deal than buying a large pin spanner as shown in the Haynes manual. To my amazement, this wrench worked like a charm, using a motor to spin an inertial weight, and a centrifugal clutch/hammer to deliver over 150 ft-lbs of torque. Several hits was all it took to break the nuts loose on both differentials. A vise was needed to supply counter-torque on one of them.

Installation is the reverse of removal steps. When reinstalling the lateral link bolt, it would be a good idea to use anti-sieze or bushing grease on all sliding and rotating parts. One of my bolts had a bushing rusted to it, and it added another 2 hours to the install. I had to remove the rear lateral link along with the lateral link bolt, break the rubber bushing, and use a Dremel cutting wheel to free the metal bushing insert/sleeve so I could re-use the large washer. If you do remove any lateral links, make note of the alignment of the 22mm inner eccentric bolt for reinstallation, or perform a wheel alignment after you're done. Reinstall the suspension, wheels, and take the car off the jack and jackstands. Torque the 32mm axle nuts to 137 ft-lbs with the parking brake applied.

You're probably not going to notice anything until you push the car hard through a corner or use heavy throttle in slippery conditions. In either case, it's nice to have a "posi" rear end.

Standard disclaimer: This information is being provided as an educational and informational service for the Subaru community. I do not make any warranties, but I will be happy to help with questions about these modifications. Please proceed at your own risk!