While researching for a potential modification (making hybrid axles), I found it very difficult to get good information on OEM Subaru CV axles. Most of the info was scattered across various forums and even then it was inconsistent. So to make heads or tails of this information I decided to put it into a table.
I will update this table as I find new information.
Cliff Notes
Starting with GD chassis, WRX rear axles (aka – 5×100/R160) have 24/25 spline rear axles. The GR chassis switched to bolt-in bearings with 27 splines so the GR WRX (08-14) has 27/25 rear axles. All STIs as far as I can tell have 27/27 rear axles. The LGT Spec B with the R180 has unique 25/27 rear axles.
| Year | Model | Axle Location | Outer CV Joint Splines (Hub) | Inner CV Joint Splines (Trans/Diff) | Shaft Size | Shaft Spline Count (Outer/Inner)* | Axle Length | Source |
|---|---|---|---|---|---|---|---|---|
| 1999 | Forester | Rear | 24 | 25 | 23mm | #5 | ||
| 2000-2002 | RS | Rear | 24 | 25 | 23mm | #1 | ||
| 2001 | Legacy Outback | |||||||
| 2002-2005 | WRX (Sedan) | Front | 27 | — | 28mm | 29? | #1, #7 | |
| 2002-2005 | WRX | Rear | 24 | 25 | 24mm | 21 or 22 | ||
| 2004 | STI | Front | 27 | 27 | ||||
| 2004 | STI | Rear | 27 | 27 | 30/30 | #6 | ||
| 2005-2007 | STI | Front | 27 | 29?/30 | ||||
| 2005-2007 | STI | Rear | 27 | 27 | ||||
| 2006-2007 | WRX | Front | 27 | 25 | #1 | |||
| 2006-2007 | WRX | Rear | 24 | 28 | ||||
| 2008-2014 | WRX | Front | 27 | |||||
| 2008-2014 | WRX | Rear | 27 | 25 | 28? | #3 | ||
| 2008- 2014 | STI | Front | 27 | |||||
| 2015-202X | STI | Rear | 27 | 27 | ||||
| 2004 | FXT | Rear | 24 | 25 | 22 | #4 | ||
| 2006-2009 | LGT Spec B | Rear | 25 | 27 |
Sources
- https://forums.nasioc.com/forums/showthread.php?t=367808
- http://wordpress.suberdave.com/?page_id=275
- https://www.wrxforums.com/threads/r180-into-08-wrx.23397/
- https://forums.nasioc.com/forums/showthread.php?t=2048240&page=8
- https://forums.nasioc.com/forums/showpost.php?p=3969011&postcount=2
- https://forums.hybridz.org/topic/73182-subaru-sti-r180-differential-and-axle-conversion-revised/
- https://thefactoryfiveforum.com/showthread.php?11112-Internal-CV-Spline-Mismatch
b.obsdian.com 
I built hybrid axles for one of my wagons so I could run an r180 with 5×100 4pot/2pots. I combined 22 spline 04-05 wrx rear axles with 06 STI inner cups using Suberdave inner races.
I had to buy a bunch of donor axles before I found some with the correct shaft spline count. Two sets I purchased were definitely oem from bugeye wrx cars and the axle shafts had 21 splines.
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I appreciate the info. I might need to do something similar fairly soon.
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