How to convert any Subaru 6 Speed (6MT) to Front Wheel Drive (FWD)

[Hobby Racer]

Some common problems people have when wanting to install a 6 speed into their 818 is finding a suitable unit that is supported by Factory Five’s FWD conversion kit. The fact that FFR only offers a conversion for model years 2006 & 2007 STI transmissions greatly reduces the number of available transmissions you have to choose from. Add to that the fact that those particular model year transmissions are highly valued by Subaru enthusiasts and you are left with high priced, often times abused, high mileage transmissions that are hard to find.

I will outline two ways to convert ANY 2001-2007 Subaru 6 speed transmission to FWD without needing the FFR conversion kit. I am not going to go over the many minor differences between each model year, or try to explain why one is better than the other. There are plenty of threads on other Subaru forums that expound on such things.

For the purposes of FWD conversion, we will group all 6 speeds into one of two categories based on the type of center differential they use. There is the Driver Center Controlled Differential (DCCD for short) and the non DCCD or viscous center differential (VCD for short). In US model vehicles, the DCCD units are found in the higher performance WRX STI cars and the VCD units are found in the Legacy Spec B cars. When looking at Japanese Domestic Market (JDM) vehicles, either can be found in all types of vehicles so its best to double check using the transmission code. The folks over at RalliSpec have compiled a chart of all the various model years and types of Subaru transmissions for your convenience.

For my 6 speed I chose a 2005 JDM 6 speed out of a Legacy Spec B.



The basic principle is very simple. Inside the 6MT there are two concentric shafts that spin independently of each other. The driven main shaft and the pinion shaft. The driven main shaft is hollow and the pinion shaft fits inside it. To force the transmission to only operate in a FWD configuration you need to lock the two shafts together so that they always spin in the same direction and at the same speed.





In either a DCCD or VCD type center differential there are always two splined parts that fit over each shaft to transfer torque between the driven main shaft and the pinion shaft. These are the parts that we will be focusing on. While this thread will be showing you how to convert a transmission that has a VCD, the concepts and techniques are the same for any type of center differential.

When converting to FWD you are trying to accomplish two things.

1. Lock the driven main and pinion shafts together in a non permanent way. You need to be able to disassemble the transmission should it need to be serviced.
2. Keep the locking mechanism in place when the transmission is rotating. You do not want your adapter sliding off when the transmission is spinning.

The next two posts will outline an easy methods and a more involved method to convert a 6 speed with a VCD type center differential to FWD.

[Hobby Racer]

This conversion is the simplest, only requiring a little disassembly, two welds, and reassembly. The downside is that it retains most of the rotating mass of the original system.

Step #1: Remove the tail shaft housing and center differential as outlined by FFR in their disassembly conversion instructions.

Step #2: Disassemble the center differential by removing the large snap ring at the rear of the unit.

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Once the snap ring is removed, push out the internals by inserting a shaft into the top of the unit and applying pressure. The viscous center section is sealed by two rubber O-rings and contains a thick viscous fluid, much like molasses. Clean up all the parts and let the thick fluid drain out of the clutch plates.





You can discard the following pieces as they will not be needed any longer.

1. Tail shaft
2. Spider gear and tie bar set
3. Plastic oil guide
4. Spacing washers
5. Bow tie slip spacer

Everything in this picture can be discarded.


If your transmission has an oil pump, it is best to remove the oil pump gear before continuing to ensure it is not damaged. Remove the snap ring and use a gear puller to remove the oil pump gear.

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The VCD housing has the driven main shaft splines machined into it. Find the part that has the pinion shaft splines machined into it.

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Step #3: Conversion

This is the meat of the conversion. You need to weld the two parts that contain the female splines together. It is easiest to weld these parts at the spots outlined in red. Be sure to get the inner piece set concentric to the outer splined section as any misalignment and the adapter will not slide onto the two shafts after welding.




Step #4: Reassembly

Once the welds are completed and have cooled down you can reassemble the center differential by inserting only the clutch pack back into the center housing and securing it with the original snap ring. If you removed the oil pump gear from your center differential, reinstall it now, remembering to reinstall the snap ring. Now reinstall the roller bearings onto the pinion shaft. I use some heavy bearing grease to hold them in place while reassembling.



Slide your completed center differential back onto the pinion and main shafts. This completes the FWD conversion. You need to follow the FFR instructions for shortening the tail shaft housing and reinstall it to the transmission to complete the job.

[Hobby Racer]

This conversion is more complex but is the lightest, eliminating all the rotating mass of the original center differential and both transfer gears. This conversion is even lighter than the FFR conversion kit as they retain the lower transfer gear.

Follow Steps #1 and #2 from the previous post to get you to a fully disassembled VCD.

Step #3: Remove lower rear transfer gear

With this method we do not need to retain the lower rear transfer gear. You can remove it by taking out the four bolts that secure it to the tail shaft housing. There is a large bearing behind the plate that is lightly pressed in. You will need to heat the back of the tail shaft housing and pry under the plate to get it out.

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You can discard everything in this picture.
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You only need the female splined section from the VCD housing and the female splined part that fits over the pinion shaft.


Step #4: Cut and prep the main shaft splined section

Cut out the outer splined section of the center differential, discard the rest as it is not used. I used a plasma cutter but any method will work, hacksaw, metal cutting wheel etc. After grinding and trimming the outer splined section is ready to be welded to the inner splined section.

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It's important to tack weld the inner and outer splined sections together on the transmission shaft to ensure they line up perfectly. Any misalignment and the adapter will not slide onto the shaft after welding. After tacking, remove from the shaft and fully weld the circumference were the two parts meet. After the welds cool you can reinstall the oil pump gear and snap ring if you removed it earlier.

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[Hobby Racer]

Step #5: Securing the adapter to the pinion shaft

The last thing to do is to keep the FWD conversion adapter you just made on the pinion shaft. While there is no thrust loading, it will be spinning and vibrating with the gear sets so it needs to be locked in place so it does not walk itself off the shafts. I picked up a 1” ID collar with a set screw from the local hardware store. Just to be safe I added 2 additional set screws so there would be equal pressure holding the collar in place. This is not strictly necessary but I general over engineer everything I do

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Now take a grinder or file and make flats where the set screws will contact the pinion shaft splines. This will help the screws "bite" and lock the collar in place.



Slide the adapter and the collar into place on the driven main and pinion shafts and tighten the set screws, be sure to use some thread locker.





If your into removing every last ounce of weight, you can cut the top of the pinion shaft off as it serves no purpose now.



Step #6: Reassembly

You need to follow the FFR instructions for shortening the tail housing and reinstall it to the transmission to complete the job.

[Larry J W]

Thanks for the tips on both the transmission and the EZ36. I Have been studying the automatic transmissions for a similar modification. It helps confirm the bearings are good enough with out the center differential. A great series of posts.
Larry

[Blu]

Thank you for taking the time to post this Hobby Racer, it was a great help.

I purchased a 2005 STI 6-speed DCCD with no reverse (broken pin) for a $1,000.
The DCCD was in great shape so i sold it for $1,500 and took the non working one as a core.
After disassembling the DCCD it was apparent the pinion shaft component would not be suitable or in my opinion strong enough for dedicated racing.
The kit came with the 5 speed conversion, it was sacrificed as it fit the pinion shaft. I cut it off at the shoulder and welded it to the main shaft component.
I did not have the extra length on the pinion shaft to add a collar, so i drilled & tapped a set screw into it.
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[Hobby Racer]

Using the FFR supplied 5MT conversion for the pinion shaft component is a great way to go. It seems to be a good fit for welding to the main shaft component.

Thanks for sharing!

[Canadian818]

Here's my little contribution. Thanks John for this thread, gave me the confidence to dive right in.

[maclonchas]

Hobby Racer,

Quick question for you on the JDM and 6 speed. I see all the posts limit the swap up to 2007. Is there any reason why the 2008--2009 JDM 6 speed that came with the EZ30 could not work with the EZ36.

Thanks

Bill

[Hobby Racer]

Hobby Racer,

Quick question for you on the JDM and 6 speed. I see all the posts limit the swap up to 2007. Is there any reason why the 2008--2009 JDM 6 speed that came with the EZ30 could not work with the EZ36.

Thanks

Bill

Nope, should work just fine. With the newer 6 speeds there is no internal oil pump though, they use internal splash lubrication.

[maclonchas]

Great thanks