STI 6-Speed Drivetrain Swap into a 2002 Subaru WRX

What a Complete STI 6-Speed Drivetrain Swap into a 2002 Needs

The complete pile is a 2005-or-later STI drivetrain dropped under the 2002 as a set. The six-speed is longer than the five-speed, the STI axles are thicker and have more splines than the WRX axles, and the 2005-and-later hubs are 5x114.3. Those three facts are why the wheels, hubs, axles, rear differential, and driveshaft move together. Stock nuts and bolts go back on. The ABS sensors and the brake lines stay.

A 2004 STI is the same job with one change: the hubs are 5x100, so the bolt pattern can stay. A 2005 case does not share axles or the rear with a 2006 or 2007. The 2004 and 2005 center differential is 1:1. The 2007 center is 1:1.1, the same split as the WRX, which is why that later box can keep the 2002 rear.

The transmission is the expensive piece. Hubs, axles, and the rear were the small used lines on a complete 2007 pile, and the brakes and a set of 5x114.3 wheels were the other large ones. Doing the labor yourself is what made that pile cheaper than a shop kit. A bare 2006–2007 swap skips the hubs, axles, brakes, wheels, and rear. A gear set in the five-speed still leaves the thin case. Builders who launched the car and downshifted it hard broke that case after a few events once the gears had already been upgraded. The six-speed is the stronger case, not only a stronger gear set.

Three Ways to Put the Six-Speed In

Pick the donor year before you buy axles. The rear ratio and the bolt pattern are decided by that year. Mixing a 2005 rear with a 2007 case, or 2005 stubs with 2002 axles, is how the swap gets stuck half-installed.

The bare path is the transmission, the six-speed clutch fork, the six-speed shifter, 2004 front stubs with their circlips and seals, and an automatic driveshaft. Hubs, struts, brakes, and the rear differential stay. That path works because the 2006–2007 rear ratio matches the 2002–2003 R160. It does not work with a 2004 or 2005 six-speed unless the rear is a 3.90.

A 3.90 R160 from a 2.2 five-speed Impreza is the cheap rear for a 2004–2005 six-speed when you want to keep WRX axles and hubs. The matching STI R180 is the rear that comes with a complete donor. Either one has to be 3.90. The 3.54 that is already in the 2002 is the wrong gear for those earlier six-speeds.

Rear Gear and the Center Differential

The rear ratio has to equal the final drive in the six-speed. The number that matches a 2006 or 2007 STI to a 2002 or 2003 USDM WRX is 3.54. A 2004 or 2005 STI needs 3.90, either that year’s R180 or a 3.90 R160. A 2004 or 2005 WRX rear was never confirmed against the 2006–2007 box, so confirm that ratio before you keep it.

The center clutch is separate from that gear. With no controller the clutch goes open. It does not lock, and the torque split itself is mechanical. A 2007 center still has a small mechanical lock when the clutch is wide open, about 10 percent. Open is fine on the street. It is worse in poor traction and on a track, and it will not hurt the transmission the way a bad rear ratio will. One shop treated a missing controller as a reason the split could wander. The installs that finished the swap drove it open, then added a controller when they wanted to set the lock.

What “open” means is not one split. One install described about 60 percent to the rear. A later install, with the controller left out, described about 35 percent front and 65 percent rear. Do not average those. If the car needs a set lock, or an open clutch when the parking brake is pulled, add a controller that has that cutout. The parking-brake switch is what opens the center clutch on a controller built with that feature.

Axles, Hubs, and the Driveshaft

The six-speed case is longer than the five-speed, so the 2002 manual driveshaft is too long. The shaft has to get shorter, and the yoke has to match the rear you kept.

• R160 stays. Use an automatic Impreza driveshaft. A 2001–2007 Impreza or WRX automatic shaft works, and an RS automatic shaft has been used in a 2002. The yoke on those shafts matches the R160.
• R180 goes in. Use the STI driveshaft. The R180 yoke is not the yoke on a five-speed or automatic shaft. An STI shaft is the wrong length for a five-speed, so do not install it until the six-speed is the transmission in the car.
• Yoke only. An STI shaft can take a WRX yoke when you are keeping the R160 and the STI shaft is the one you already have.

Front axles follow the stubs in the case. 2004 STI stubs match 2002 and 2003 WRX axles. 2005-and-later stubs do not. Those 2004 stubs fit in a 2006 or 2007 case, with the 2004 circlip and the 2004 seal. They do not unbolt out of a WRX five-speed. The WRX stubs stay in the five-speed unless the transmission is taken apart. Six-speed stubs come out of the six-speed.

STI axles fit STI hubs only. The shaft is thicker and the spline count is higher, so they will not go into a WRX hub. Keeping 5x100 means keeping WRX axles, which means keeping the R160, unless the rear hubs change to 2004 STI hubs for an R180. On that 2004 path the front hubs match the 2002, and the rear hubs have to change because of the R180 splines. 2005-and-later hubs are 5x114.3. 2005 axles are not the 2004 axles, and they are not the 2006–2007 axles.

The WRX rear-differential support bracket fit an R180 on a 2004 six-speed installed in a 2006 WRX. The 2002 crossmember took an STI rear and STI axles on a wagon once the hubs matched. Plan on the six-speed crossmember when the car is an automatic and does not already have the manual one.

Speed sensor

Plug the vehicle-speed sensor in before the first start. A low circuit sets P0502. The STI sensor is a 3-wire plug. An automatic sensor is 4-wire, so a 4EAT-to-six-speed car needs the 3-wire side tied into that harness. A retainer left in the sensor bore will let the plug sit on the threads without latching. One automatic car kept its automatic ECU and harness. ABS and cruise still worked. That ECU can set P1596, and the code does not change how the car drives. The original ABS sensors are reused on the knuckles you install.

Clutch, Fork, and Shifter

The six-speed needs the STI clutch fork. A WRX fork will not release it. The disc, the pressure plate, and the flywheel have to be one set. A 2002–2005 WRX set works. A 2004–2007 STI set works. The STI disc is a larger diameter, so it does not belong on a WRX flywheel, and a 2006 STI clutch will not bolt to a 2002 flywheel. A 2006–2007 WRX clutch is the other style from a 2002–2005 WRX and from an STI through 2007. Do not mix that set in.

A 1998 RS uses a different clutch and is outside this list. An automatic also needs the clutch pedal and the line from the master to the slave. Those hydraulics are the same whether the transmission that follows is a five-speed or a six-speed. The six-speed mount comes on a used transmission. The hardware that held the five-speed is the hardware that holds the six-speed.

Reverse lockout and the boot

The shift linkages follow the six-speed pattern. The lever has to be the six-speed lever, because the reverse lockout ring lives on that lever, not on the cable. Under the ring is a cone that the boot has to pass. A 2004 STI boot has a button so it opens, clears the cone, and closes under the lip. That boot fits a 2002–2003 console. A later STI boot needs the later center console. A WRX boot can be pulled over the cone and fits poorly.

Replace the reverse-light switch and the clutch-pedal switches while the transmission and the pedal are out. A bad reverse switch is a small part that only shows up after the car is back on the ground.

Brakes, Suspension, and the 1999–2001 RS

On the bare 2006–2007 path, the struts, springs, hubs, and brakes stay. The moment STI axles go in, STI knuckles go in. STI knuckles need STI struts and springs, and they use STI backing plates, which means the Brembos. There are mixes that avoid pieces of that chain. The complete list is the one that does not depend on a mix.

• 2004 hubs. The 5x100 knuckles match, so the 2002 struts can stay. Rear hubs still change if the rear is an R180.
• Wagon arms. STI lateral links bolt to STI knuckles and to the WRX crossmember. Stock WRX links and the WRX rear bar also bolt up. STI links need the I-shaped STI end links, not the C-shaped WRX links. A wider STI front bar needs STI end links and replaces the WRX bar.
• Wider track. STI front arms and rear links push the tire out. They are optional. On one wagon they looked flush and rubbed after the fenders were cut, on 27 mm bars front and rear.
• STI struts on a wagon. They bolt to the wagon rear perches and to 2005 knuckles, and the wagon sits low on STI springs. Struts and springs do not require new arms.

A 2007 STI suspension in a 2002 left the parking-brake cables about an inch short of the lever under the console. The lever cable from that STI, part 26051SA050, is about 1 to 1.5 inches longer than the WRX cable and made up the gap. The new cable may only have room for one jam nut until the handle is adjusted so the brakes are not dragging.

What the RS adds

A 1999–2001 2.5RS takes the same drivetrain list as the 2002, plus the rear subframe the differential and axles mount to, and a pull-style slave from an STI or a 2002–2005 WRX. The clutch master stays. ABS sensors, brake lines, and the bolts stay. STI lower control arms fit. STI rear lateral links do not: camber goes off. Use adjustable links, the STI links made for the RS, or the stock links. Stock-style rear links limit the rear bar to about a 22 mm bar.

Front strut mounts are the same from 1993 through 2007. Rear STI mounts are larger. Two of the three studs line up. The third stud has to move, or the hole in the perch has to be opened into a slot. Coilovers that use an STI knuckle and an RS top mount skip that. STI wheels can go on, and a 225 on those wheels is likely to rub. Plan on a different tire, coilovers, or fender work before the car is lowered.

Swap Checklist

Finish the match on the ground, then put the transmission in. A six-speed is tighter against the tunnel than the five-speed it replaces, and the clutch is harder to reach once the case is up.

1. Name the donor year. 2004 is 5x100 and 3.90. 2005 is 5x114.3 and 3.90, with its own axles. 2006–2007 is 3.54 and can keep a 2002–2003 R160.
2. Match the rear. Keep the 3.54 R160 only behind a 2006–2007 six-speed. Use a 3.90 R160 or the matching R180 behind a 2004–2005. Read a 2004–2005 WRX rear before you trust it.
3. Match the driveshaft to that rear. Automatic Impreza shaft for the R160. STI shaft for the R180.
4. Match the front stubs. 2004 stubs, circlips, and seals if the 2002 axles stay. Full STI axles only with STI hubs.
5. Install one clutch set and the STI fork. 2002–2005 WRX parts together, or 2004–2007 STI parts together. Add the pedal, master, slave, and lines on an automatic. Add a pull slave on a 1999–2001 RS.
6. Fit the six-speed lever and a 2004 boot on a 2002–2003 console. Change the reverse-light switch and the pedal switches while they are easy to reach.
7. Leave the center clutch open, or add a controller. The factory controller needs the STI ECU. Plug the speed sensor in. It is 3-wire on the STI.
8. Follow the knuckles if you change them. STI knuckles bring STI struts, springs, backing plates, and Brembos. On an RS, add the rear subframe and fix the rear top mount. On a 2007 suspension in a 2002, plan on parking-brake cable 26051SA050.

FAQ

What does a complete STI 6-speed drivetrain swap into a 2002 WRX need?

The six-speed with its fork and mount, a matching R180, the STI driveshaft, STI axles, STI hubs and knuckles, Brembos, 5x114.3 wheels, and the six-speed shifter. ABS sensors, brake lines, and the original bolts stay. A 2006–2007 box can skip that chassis list and keep the 3.54 rear, the 2002 axles, and 2004 front stubs.

Can I keep my 5x100 wheels?

Yes, on the bare 2006–2007 path, or with a 2004 donor. STI axles from 2005 and later need 5x114.3 hubs. A 2004 R180 still needs 2004 rear hubs, because the R180 splines do not match the WRX rear hubs.

Do I have to run a DCCD controller?

The car drives with the center clutch open. Lock is what the controller adds. The factory controller needs the STI ECU and the sensors that are not part of a transmission swap.

Will my 2002 clutch work?

A 2002–2005 WRX clutch, pressure plate, and flywheel work as a set, with the STI fork. The slave and master on a 2002 can stay. The STI disc is larger and needs the STI flywheel.

Does the same list fit a 2003?

Yes. The 2003 uses the 2002 parts list, including the 3.54 rear behind a 2006 or 2007 six-speed. A 1998 RS does not. Its clutch is a different style.

What is different on a 1999–2001 RS?

Add the rear subframe, a pull-style slave, and a fix for the larger rear STI top mount. Leave the STI rear lateral links off unless they are the RS versions or adjustable links. The clutch master stays.

Do I need Brembos if I keep the 2002 hubs?

No. Brembos follow STI knuckles and STI backing plates. The bare 2006–2007 path keeps the 2002 brakes. A 2004 R180 still needs 2004 rear hubs, and those hubs are 5x100.

Can STI axles go in the 2002 hubs?

No. STI axles are thicker and have more splines, so they need STI hubs. The 2002 front axles stay when 2004 stubs are in the case. The 2002 rear axles stay only with the R160.