Fitting a 6 speed to your Impreza WRX STI, Forester, Liberty, SVX or Outback

Fitting a 6 speed to your WRX, Forester or Liberty

Six speed manual Gearbox conversion requirements:

There are a few different methods and trains of thought about fitting a 6MT gearbox to your WRX, Liberty, Outback or Forester.

Generally they require the same parts with a few exceptions. To simplify it and remove a few misconceptions we have created this blog to help you.

The main reason of fitting a 6MT STI gearbox is to replace the much weaker 5MT boxes. Even though they are considerably more expensive up front in the long run could end up being the cheaper option after having to replace five speed after five speed in the quest for more power and reliability.

The Subaru WRX STI 6MT was first fitted to the JDM MY01 Bug Eye STI as a standard fitment. The box continues through to today’s STI’s. They can also be found in the Liberty GT and 3RB and Forester STI ranges.

All 6MT gear boxes  are around the same weight and dimensions as each other but they have continuously revised the gear ratios, front diffs, driver controlled centre diff feature and the later models have gone away with the oil pump but have included better DCCD for the STI models. Most 5MT weigh around 65 kg and the 6MT weighs around 85 kg.

The transmission code can be found on the transmission, this will explain the internal components and define which model it came out of and the gear ratios and differentials and DCCD type.


A Must when fitting a six speed:

The 6MT gearbox, prerrably a low km gearbox (don't confuse STI / Liberty 6MT gearboxes with the new 6MT's found in the diesels and newer WRX's, those gearboxes are as weak as per the original 5MT).
The 6MT STI style clutch release fork (different to 5MT boxes)  -  
6MT STI style shifter assembly with the reverse lock out cable (note there are two different types which change the length of the shifter assembly).

It's also wise to ensure you have new spigot and throwout (clutch release) bearings, failure of either can be catastrophic to your new gearbox.

Clutch Release Bearing:

Spigot Bearing:

Note that the 6MT tail shaft has different splines to the 5 speed WRX gearbox also is shorter in length. However an auto tail shaft shares the same splines and is the same length required. You will also need to ensure you match the rear diff ratio match to what is required for the 6MT you are fitting. It must be noted that when using the auto stubs, the correct oil seals must be used or the gearbox will leak.

6MT internals and seals:

Lastly a qualified or experienced mechanic to carry out the conversion.

We try and stock most of the items to assist you with your DIY upgrade or can complete it for you at a great price with absolutely confidence in the gearbox and work completed by us.


Cross members:

It’s important to fit the correct cross member to the correct car as the wrong one will cause vibrations in the drive lines as the drive line angles will be too extreme for the tail shaft to rotate in harmony. The rear rubber mount on the 6MT is a full rubber insert which helps strengthen the driveline rigidity and stop the unit from bouncing around. This mount can be fitted to various cross members as they share the same bolt pattern. Generally it’s the Foresters that require the standard cross member for the STI box as they have spacers fixed to it.  


Tail shafts:

As previously mentioned the six speed and auto tail shafts are shorter in the front section (first half before the centre bearing) than the 5 speed box. The main difference in the shafts is that the six speed shaft has a universal joint just after the centre bearing whereas the auto shaft has a constant velocity joint (slip joint). On the later model 6MT speed STI’s (06 and newer) the rear diff pinion flange on the tail shaft is larger and will not mate up to the standard R160 diff flange as previous models.


Rear Diff:

The rear diff will have to suit the 6MT you purchase, the transmission code will help determine which ratio you will have to match. The only diff ratios in the rear are a 3.9 or a 3.54, The 3.9 ratio was used until mid 2005 where than changed to 3.54 rear diff ratio. This is also when the larger pinion flange was fitted however this can be replaced with a standard sized flange.

There are two rear diff sizes in the Subaru range, R160 and the R180. The R160 is a standard fitment across the board with multiple variations in ratios, The R180 is generally an STI Fitment and takes larger sized rear drive shafts on both the hub and diff sides.

Here are some of the items you’ll need to complete the conversion:

6MT Slave Cylinder:

6MT Starter Motor:

6MT Flywheel bolts:  

6MT Flywheel:

The standard 5MT clutch, flywheel and starter motor can remain and be used, however the STI 6MT clutch has a larger surface area on the clutch pressure and friction plate and the flywheel is lighter in weight than the 5MT. If you use the 6MT set up you will require more clutch bolts. The 5MT slave cylinder can remain.

Here is some data on the clutch pressure plate clamp loads from factory, this data is found in the Subaru technical manuals:

RS Liberty - 620 kg clamp - 5MT
MY92-96 WRX = 700 kg clamp - 5MT
MY02 WRX = 800 kg clamp - 5MT
MY02 STI = 830 kg clamp - 6MT
MY06+ STI (2.5L) = 930 kg clamp - 6MT
STI Competition Upgrade = 1050 kg clamp - 6MT

Clutches and clutch kits:


In General:

The 6MT set up will depend on the budget and application,  Any 6MT STI gear box will bolt up to an EJ, EG and EZ range of Subaru engine,  The different models of 6MTs should be considered when purchasing to maximize the compatibility to your application, below are some points for a guide.

  • Front diff, male or female ends on gearbox side of drive shafts, this will save you replacing them if you choose the correct style six speed.
  • Diff Ratio: most likely replacing the rear diff ratio is a must however the 06 sti 6MT has a 3.54 rear just like the 99 aus delivered wrx, so replacement is an option then.
  • DCCD: A great factor on the later models, however not a must for wiring in , can be left disconnected and the transmission still perform correctly only with more bias to the rear wheels!

The fitment should be carried out by a qualified technician that has experience in field. This information should only be used as a guide only.