Here is a more complete write-up for the Subi diff and axle swap, and it's in it's own thread so it'll be easier to find.
If you come across a Subaru STi differential and axles and are considering swapping them into an s30 like I did, here is what needs to be done:
Before spending any money I made sure that this setup was even remotely possible. Because I planned on using the STi axles with the STi Diff, I knew there were no issues on the axles-to-Diff side, with splines and what not. his is because the Axles and differential are from the same year (2004) STi. I also noticed that the input flange, for which the driveshaft connects, on the differential was a different bolt pattern. But this problem can be easily resolved by simply swapping the r200/r180's input flange (from the datsun's) with that of the Subi.
Note: I used the r200 flange and needed to do some trimming on some of the inside dust shield in orded to prevent contact with Subi differential front seal.
Now there were two big issues that I came across. One: Is that the Subaru axles are about 4-6 inches to long. Two: Is that the hub side(outside part) of the subi axle has splines that go into the STI's wheel hub assembly. And I needed to somehow modify it to a flange.
Picture 1: is a Picture of the Subaru axle, without the end CV, going through s30 hub.
Picture 2: Is outer Subaru CV case, with 280z stub axle flanges.
1. For the length issue, I thought, there had to be a place that made a shorter center shaft for an STI. Sure enough, The Driveshaft shop make a shorter center shaft for the STI. The stock shafts are 20 7/8". And after measuring and calculating I determined that the ideal length would be 15 1/4" to 15 1/2".
For my Calculations: Since I won't be using the stock center shafts, I cut them in half with the CV cases still on. Once I FINALLY cut through, I snapped in the inner half into the differential, then the outer sat into the s30 hub hole, similar to Picture #1 above. Then let the two axles cross, and then I make a mark on each one. Like this:
EXTREMELY IMPORTANT: The center shafts of the Subaru axles have different combinations of splines on either side. I don't know why Subaru did this, but pay attention to this so you don't get the wrong center shafts. For my 2004 axles they are 30-30. Meaning 30 splines on each side. (Remember this is the spline count for the centershaft.) Other combination can be 32-34 for example. So if your ordering these from The Driveshaft Shop, make sure you know what yours are by COUNTING! Then let them Know you desired length and the spline ratio/combination/number, and your good to go.
As a bonus, these new shafts are built handle 400 Hp +. Now I'm just running a L30 stroker, so there is no chance I will break them. Oh I should mention these new shorter center shafts were about $400. That also includes: 4 New CV boots, 4 Tubes of CV grease, and 12 clamps. (6 small, 6 large).
Here the stock shaft and new shaft are side by side:
2. For the other issue, the problem with not having a flange. I took the outside axle case and the 280z stub axle flange to a machinist. I told him that I wanted the spline section removed and a flange welded on to match the 280z flange. Like this:
Notice the yellow ring on the drawing, this is used to line up and center this flange to the Stub-Axle flange. Same as the stock design. I made sure to include this with my notes to the machinist.
Here is some of the initial planning, with the Stock outer CV case and 280z Stub-Axle flange
And this is the outcome, notice the outer CV case (yellow) with it's new flange welded up and painted. The red circle shows the little grooves I had to make in order for the bolts to fit through the flange.
These are the axles in pieces and ready to be assembled:
Well once you put the new CV's together your pretty well all done. Here is a picture of them in the car after about 3000 miles:
Works flawlessly! The axles fit perfectly so it isn't a PITA to take out, like it was with the z31 axles. But best thing about this setup is that I got Limited Slip, the 3.90:1 gears that I wanted, both my axles and Differential have less then 20,000 miles on them, and it's lighter!
-The 2004 STI LSD differential with 3.90 gears. (16,000 miles) cost me about $360.
-Both the axles (18,000 miles) cost about $280.
-The new center shafts, and boots. etc. cost $400
-Welding/machining on outer CV case cost about $100
-Some POR paint and Redline gear oil. Under $100
While it cost a little over $1,200. Spending $700-$900 on z31 r200 LSD is sort of ridiculous. Considering how many miles there are on a 1987-1989 differential. Maybe a couple of years ago the r200's ran for a lot less than that. But I am very please with my setup. And I would highly recommend it.
I hope this helps those considering this swap.