Want to 6 Speed swap your Subaru? Maybe you are tired of breaking the weaker 5MT, or just want an extra gear for a better driving experience. Here’s what you’ll need.
Fitment and required parts vary depending on your chassis and your donor gearbox. Use this as a guide of what parts you may need.
TY856 6-Speed Gearbox
Check its condition and replace any leaking seals before installing into the car. Research the gear ratios of the transmission you are intending to buy to compare to your current transmission. Some models are short ratio (providing faster acceleration) while others are much longer, leading to less revs and a more comfortable ride when driving on the highway. All TY856 6MT gearboxes have similar torque capacity which is much higher than the TY754 5MT.
Differential
A rear differential which matches the transmission must be used. The front and rear diff must have the same ratio. Ensure your donor gearbox and diff match.
Important Note: Some transmissions use a 1.1:1 transfer gear in the transmission and therefore the rear diff ratio must be multiplied by 1.1 before comparing to the front diff. For example, a transmission with a 3.9:1 front diff and a 1.1:1 transfer gear requires a 3.545:1 rear diff (3.545 × 1.1 = 3.9).
Clutch and Flywheel
The TY856 6MT uses a pull type clutch, compared to the push type that is used on most 5MT TY754 transmissions. A 6MT pull type flywheel and matching clutch is required when installing a 6MT in your vehicle. Both EJ and EZ engine flywheels are cross compatible.
Clutch Slave and Master Cylinder
To go with the pull type clutch a matching 6MT slave cylinder is required. It is best to also swap over the master cylinder with the slave so the piston/bore sizes are matched.
Starter Motor
A 6MT starter motor is required.
Tailshaft
The 6MT is a longer transmission compared to the 5MT. This means a shorter tailshaft is required to accommodate the extra length. A 6MT driveshaft to match the chassis can be sourced, however an easier option is usually to buy a tailshaft from a 4AT vehicle. The 6MT and 4AT transmissions are the same length and thus the tailshafts are the same length as well. Ensure you purchase a 6MT or 4AT tailshaft from the same model as yours. For example, if you have a GD Impreza make sure the donor one is also from a GD.
Shifter and Linkage
A 6MT shifter linkage to suit your chassis is required. This can be sourced from a factory 6MT vehicle or Subaru also sells STi short shifters brand new to suit most chassis.
Mounts and Crossmember
The original 5MT crossmember bolts up to the 6MT transmission and fits in the same location. In the case of a factory 5AT car, the bolts are often in a different place in the chassis and will require custom brackets.
A 6MT rear mounting bush is required as it is not the same as the 5MT – usually this will come with the donor gearbox.
Axles
The original front axles from your vehicle will work without modification in most cases. However, if your vehicle had stub axles in its original transmission and your donor 6MT is from a 2004+ then you will require stub shafts to be installed into the TY856. Conversely, if you are installing a pre-2004 transmission into your car which did not originally have stub axles, these will need to be removed from the donor transmission prior to install.
The original rear shafts from the vehicle will also work provided you are putting the same size diff in. For example, original diff is R160 and donor is R160, or original diff is R180 and donor is R180. If you are upgrading from R160 to R180 then either hybrid axles (dismantling and combining factory axles) or custom axles from a shop are required. If you are going from R180 to R160 then some standard axles from your chassis should be easy to find.
Speedometer Signal
2004+ Liberty and 2008+ Impreza 6MT transmissions no longer have a vehicle speed sensor (VSS) equipped. To use one of these transmissions in an earlier non-canbus vehicle the transmission must be disassembled and machined to install a speed sensor. Alternatively, a much easier option is to use the ABSvss speed sensor module to supply the speed signal. See here for more information.
Reverse Switch and Neutral Switch Wiring
The reverse switch and neutral switch may need a wiring adapter depending on your specific swap. If this is not connected then the car will work however the reverse lights will not work. It is also recommended to replace both the neutral and reverse position switches as these commonly go bad and are a pain to access with the transmission in the car.
Gearbox Oil and Fluids
It is recommended to drain and fully replace the gear oil in the transmission. Castrol Syntrax is recommended for Subaru 6MT transmissions. Rear diff gear oil can vary depending on LSD type (Open, Viscous, Plated or Torsen).
Disclaimer
The information provided in this guide is offered in good faith and is intended for general reference only. Fitment, required parts, and compatibility vary depending on chassis, model year, and donor gearbox, and no warranty, express or implied, is made as to its accuracy, completeness, or suitability for any particular vehicle or purpose. Always verify part compatibility and consult a qualified mechanic before undertaking any modifications. Any reliance you place on this information is strictly at your own risk, and the author accepts no liability for any loss, damage, or injury arising from its use.