Differential/Rear End Information

[Splinter]

After spending a few days pouring over part numbers, taking measurements of axles I have, and comparing with photos online, Ive come to a few conclusions. I cannot 100% verify all these facts, but they may be useful to people considering a rear end swap.


There are 3 basic types of axle

Type 1: Female inner joint - USDM Legacy Turbo 91-94, USDM WRX 02-05
Type 2: Narrow male inner joint - JDM Legacy Turbo 91-94, USDM N/A cars (impreza, legacy, forester) 95-01
Type 3: USDM STi 04-05

These each corrospond to a type of differential

So, direct bolt-on as far as axles go (not accounting for gear ratio or mounting) would be:
91-94 USDM Legacy Turbo
02-05 USDM WRX
Which pretty much puts a huge restriction on the availability of LSDs

If you swap out the rear hubs to RS rears, it opens up the following:
JDM Legacy Turbo 91-94
USDM Impreza
Legacy GT 97-01

There are quite a few options for LSDs in here, as some RS's (00, 01 option) came with an LSD, as did LGTs

The above can also be accomplished by dismantling RS axles and attaching Legacy outer joints

In order to swap in a USDM STI rear diff (R180 LSD, for instance), you need to have STI inner joints. I have not seen any other inner joints that are similar, but I am going to be looking into the possibility of 300ZX inner joints, since they use the same diff

So if you have a set of STI axles, you can take off the outer joint, put on Legacy outer joints, and you're laughing.

I'm going to keep adding to this as I get more info, corrects and additions are appreciated

[555BCTurbo]

but I am going to be looking into the possibility of 300ZX inner joints, since they use the same diff

Pretty sure my buddy's 92 300ZX has an R200

[mike270]

I'm still a bit confused about this. From what I've seen in the service manuals, the 02WRX's had the male inner joints like the early Impreza's, Foresters, etc.

Is this then a valid assumption that if I have the correct end joints that the lengths will be correct? It sounds like lengths of the rears have been constant over the years? I think WRX wagon fronts are longer than sedan fronts, but wasn't sure if that kind of translation carried over to rear axles or Legacy's or other Subaru models or if the WRX wagon was the oddball.

Also do I read it correctly that the hubs are different on the T-Legs? Is the spline count on the hub different than the WRX / early Imprezas? If so, my T-Leg hubs, and 03 Forester diff combo will be unworkable without custom axle work?

[Splinter]

Actually now that I look at my info, the WRXs might have male inner joints...

It didnt occur to me that the WRX axles I have could be front axles from a wagon

All axles I have are the same length. WRX & STI sedans had a wider track, so the axles should be about an inch longer. All the rest should be the same length, although the tribeca is probably different.

The Impreza axles I have are the same length as the Legacy axles.

[mike270]

Cool, thanks for the info.

Do you happen to know about hub fitment / spline counts? I thought the Legacy hubs were the same as other GC hubs, so in your first post i would think that if you upgraded to an 01RS LSD that a RS axle would fit properly with the stock T-leg hubs (assuming equal lengths).

Also, what would be the easiest way to make sure the axle length is correct (without breaking one first)? They should have some free play side to side I would assume even when up in the air (certainly when loaded on the ground I would think too given thats the shortest axle distance), but how much free play should there be?

[Splinter]

The RS axle is at least 3mm narrower than the legacy axle

[mike270]

3mm in length? That doesn't seem like much to me. I would have expected there would be ~.5-.75 inches if there were a difference. Could that 3mm be variation in bearing position or something along those lines or are you measuring at full expanded or fully compressed positions?

[Splinter]

narrower.

[Splinter]

Impreza on the left, Legacy on the right

[mike270]

What do you mean by narrower? What measurement is narrower? Based on the pic I'm guessing 3mm smaller diameter at the hub splines...ie Impreza axle too small for Legacy hub?

[Splinter]

Correct.

[mike270]

Interesting. Can anyone who has done this before confirm this before I go buy new hubs as well? I'm guessing there have to be others that have do this sort of swap before.

[Splinter]

You dont need new hubs

Just swap the Legacy outer joints onto the forester axles

[mike270]

You are refering to replacing the hub side axle joint on a RS axle with the legacy one correct? Having never ripped into an axle, only swapped whole units out, is that a simple DIY thing? Just pick up some new CV grease?

[Splinter]

And some band clamps

[mike270]

Looking at the 02 service manual, left and right axles are different lengths as well, marked by notches in the shaft.

02 wagons are shorter than 02 sedans by 10mm (regardless of engine), non-turbo are longer than turbo by 5mm (regardless of body style), and left is shorter than the right axle by 10mm.

So its much more complex than just 3 types of rear axles...

[Splinter]

axle length doesnt really matter all that much imo

a 1 inch change in suspension height moves the axle position by at least 10mm

I dont see how the right and left could be different though

We only sell one part number for aftermarket WRX axles at my work, and subaru claims to have a symmetrical drivetrain

are you sure you're interpreting it correctly?

[n2x4]

He is correct, they are different. It didn't make much sense to me either, but I found out all about it when I put my 05 WRX drivetrain in. Here's some of what I found:

WRX front axle differences-
2 versions of front axles, male and female inner joints. I don't remember what year was the cutoff before they switched to strictly male inner joints, but earlier models (02/03) did have female front axles like our legacy.

Rear axles-
There is a right and left axle. There also are wagon and sedan axles. All WRX rear axles have male inner joints.
As far as size differences go, I don't know exact measurements, but the left axle is the shorter and the right axle is the longer one.
The wagon long axle is = to the sedan short axle.
I also confirmed this with Innovative Tuning as they've dealt with the same issue (wrx swap into legacy)

So basically front axle size is equal, but rear axles are different. Only thing I could think of was an offset of the rear pumpkin to the left, which would definitely cancel out symmetry. I know that this thread is targeted towards rear drivetrain discussion, but I figured I'd throw out some front data while we were at it.

[mike270]

He is correct, they are different. It didn't make much sense to me either, but I found out all about it when I put my 05 WRX drivetrain in. Here's some of what I found:

WRX front axle differences-
2 versions of front axles, male and female inner joints. I don't remember what year was the cutoff before they switched to strictly male inner joints, but earlier models (02/03) did have female front axles like our legacy.

Rear axles-
There is a right and left axle. There also are wagon and sedan axles. All WRX rear axles have male inner joints.
As far as size differences go, I don't know exact measurements, but the left axle is the shorter and the right axle is the longer one.
The wagon long axle is = to the sedan short axle.
I also confirmed this with Innovative Tuning as they've dealt with the same issue (wrx swap into legacy)

So basically front axle size is equal, but rear axles are different. Only thing I could think of was an offset of the rear pumpkin to the left, which would definitely cancel out symmetry. I know that this thread is targeted towards rear drivetrain discussion, but I figured I'd throw out some front data while we were at it.

Yeah, I got basically the same thing from the manual too. And to add further complication, the axle diameter changes depending on model, and engine type (or maybe its just model, can't remember right now). Not much, I think it only changed 1mm, but still, Subaru didn't want to make it straight forward for some reason.

Without the rear diff in front of me, I think that the rear pumpkin is in fact visbly offset towards the left side of the car, but I had figured that offset was accounted for on the trans side as well, but apparently not.

Personally, I think axle length is a very important feature, at least in my application (rally). With all the time at speed in the air, I don't want to be over-extending an axle when the suspension droops and then land and snap it in two because I didn't take length into account.

[mike270]

Also, I'm going to be calling a local Suby axle rebuilder today and ask about hub diameter differences. Changes in the hub size by 3mm still doesn't quite make sense to me since I know a few people that have swapped Turbo Legacy rear hubs into their GC's to allow for the vented rotor (or just upgrade to disc brakes), and I didn't think any of them had to change axles out to do it.

I'll post the answer later today.

[Splinter]

Well to be fair, this thread is targeted more towards diff swaps, and what axle joints to use with what diffs

[Innovative Tuning]

I'm using a WRX rear end with legacy turbo 3.9 ring/pinion and back plate (so we know it's 3.9) with WRX rear axles, sti driveshaft, sti tranny, sti stub shafts (front) with some other circlips, and legacy turbo front axles.

This combo works well for anyone putting an STi trans. in a legacy SS.

[mike270]

So you are using stock Legacy hubs with the WRX axles then, assuming I read that correctly? Or does that include WRX rear hubs?

How do the WRX axles seem as far as length? Do they seem approximately where they should be? (i.e. - They don't seem over extended when the car is in the air or too long when on the ground)

That seems to agree with what I heard from the axle rebuilder. He was saying that the hub bearings should all be the same until it came to the STi, which he thought had a different bearing size then others, which would seem to make sense.

[mike270]

Looking again at the service manual, it looks like the stock male axle stubs simply slide into the diff like the newer WRX female diff setup.

Seems to me that given the male axle stubs are held in by circlips just like the WRX setup, that one could take the male stubs out of the stock SS diff and put them in a WRX diff and not have to change axles at all.

Anyone tried this, does it work or are there still internal differences?

[Splinter]

Ill try taking mine apart tomorrow