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#1: 08-14-2010, 01:48 AM
NSFW's 6MT swap thread
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I typically create threads like this after the stuff gets installed, but I'm making an exception in this case because I need a place to take notes while I sort through the information that is sprinkled throughout the "6mt master swap thread" (and others) so that the next person to undertake this swap won't have to go on an easter-egg hunt to learn what they need to know.

There are two main recipes for 6MT swaps. They key difference between them is the rear diff ratio. Both use 3.9:1 front differentials, but the later transmissions have a 1.1:1 ratio on the rear output shaft, which requires a 3.54:1 rear diff.

Recipe A - 3.90 rear diff
Transmission options: 04/05 STI*, or 08/09 Spec B
Rear diff options: 04/05 STI, possibly 07-09 Legacy GT R160**
Rear axles: 07-09 Spec B (axles 28421AG03B, axle seals 806732200)
Front axles: Regular LGT***
Driveshaft: use a Spec B driveshaft, and replace the differential's input flange with the R160 flange.
(The Spec B R180 has an R160 input yoke, but is only available in 3.54:1.)

Recipe B - 3.54:1 rear diff
Transmission options: 06+ STI (I don't know what the upper bound is, but unclemat used an 08)
Rear diff options: 02-05 WRX R160, 07-09 Spec B R180*****
Rear axles: Regular LGT with R160, Spec B axles with R180.
Front axles: Regular LGT
Driveshaft: Spec B****

Note that there are two versions of the Spec B 6MT transmission and they have different final drive ratios.

* Some 04 STI transmissions need minor changes for LGT front axles - click here for more info.
** I have conflicting information about which Legacy GT R160s have which ratios.
*** 04 6MT will need new axle seals to use LGT front axles. (Part numbers here)
**** Per Z0rro, an 05-09 4EAT driveshaft can substitute for a Spec B driveshaft.
***** says the 08/09 Spec B uses a 3.9 final drive, however that apparently includes the 1.1 reduction in the transmission.

Small parts: (needed for either recipe)

Please confirm these part numbers for your particular transmission.
Most of these are just what I used for my 05 STI 6MT swap; later years may have differences.

Keep your 5MT neutral sensor switch, put it in the 6MT.
6MT shift linkage (Preferably Spec B; the Impreza shift linkage is shorter) (Is this still true for 08+ Impreza?)
6MT clutch fork - 30530AA050
6MT clutch release shaft - 30532AA020
6MT clutch slave cylinder - 30620AA172 - alternate:30620AA173
6MT clutch hose
6MT throw-out bearing
6MT rear transmission mount (same part for Spec B and Impreza)
Reverse lockout cable - 35060AG010
Downpipe support bracket - 44021FE030
R180 diff mount from the spec B - 41310AT040 (not strictly necessary, but click here for some info)
Clutch fork shaft plug (threaded cap that holds the fork shaft in) - 30550AA000 for 04-07, and 11021AA010 for 08+
Output Shaft Oil Seal (oil seal for the driveshaft interface) - 806735210 - $11.50
Clutch fork spring (spring that connects clutch fork to bell housing) - 30546AA060 - $5.20
Inner shift boot (foam/rubber, mostly for noise insulation, you can reuse the 5MT if you trim it)
If you get a DCCD diff, you'll need an aftermarket DCCD controller.

Courtesy of ShockerGT:
If you have an STI shift linkage, the shift lever will not be centered as the STI shifter rod and shifter stay are shorter than the Legacy parts. So, you'll need these:

Spec B Shifter Rod: 35041AG020
Spec B Shifter Stay: 35031AG020

Stick with 04-07 STI shift levers, as the 08+ versions are not compatible.

What you DON'T need:

Vehicle speed sensor - the LGT speed sensor is not on/in the transmission.
6MT rear transmission crossmember (same part for 5MT, just needs the 6MT mount (the rubber part))

Considerations: (which recipe is right for you?)

The Spec B 6MT has a 50/50 torque split, lacks DCCD and has an open front diff, but is less expensive
04/05 STI 6MT has a 35/65 torque split, open center diff, and DCCD
06+ STI 6MT has a 41/59 torque split, helical center diff, and DCCD
R180 rear diffs are stronger than R160 rear diffs.
The Spec B R180 uses an R160 input yoke. Not sure how much this affects strength.
Unclemat opines on differential technology.
And so does Homemade WRX.

Last but not least, the Spec B 6MT gear ratios are more evenly spaced than the USDM STI 6MT ratios, and they're a bit taller. Rallispec will install them into STI transmissions, and they call it a "high speed" gearing option. I did it mostly for the even spacing. Better mileage would be a nice side-effect if I stayed out of boost, but how likely is that?

Reference material:

Hammer Down's thread:
sti_lusts's thread: 6MT swap thread:
BDII's NES 6MT swap thread: 6MT swap thread:
An Impreza 6MT swap:
A thread with some DccdPro info:
Info about differential types:
Spec B shifter assembly:

Part sources:

Millenium Auto - (Used Subaru parts, including 6MT transmissions)
Silverline Automotive - (6MT swap packages)
Thrash - (custom driveshaft, $650ish)
Spiider - (Aftermarket DCCD controller)
SGP - (Axles, seals, clutch fork, etc)

Instructional video:

Last edited by NSFW; 02-28-2012 at 12:04 AM..
The Following 4 Users Say Thank You to NSFW For This Useful Post:
BlackHole (01-09-2013), Dujo (09-08-2011), Juventas (05-30-2011), the kev master (06-05-2013)
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