I installed a 2006 STi transmission into my 2003 WRX, using early 2004 STi axle stubs, and was leaking transmission fluid at a pretty rapid pace. Yesterday I found out why...
There are two different types of axle seals which apply to different model year axles, and I have listed them below.
Early 2004 Axle Seal Part Numbers (to be used with axle stubs / female axles):
Late 2004-Present Axle Seal Part Numbers (to be used with male axles):
Both seals have the same outer diameter and will fit into any transmission, however the inner diameter where it seals with the axle is 30mm for the old vs. 36mm for the new. This is because you will notice that on my 06 STi front axle, there is actually a step up where the axle gets significantly larger right before the green cv boot case.
I dont want to promise that a WRX or STi or certain model year will do what, but the info I have is that the quoted part numbers apply to WRX or STi build dates up until Aug 2003 (includes early 04 models). The latter numbers I listed apply to anything from then on for the STi (and possibly WRX too? not sure...)
And because it may also be helpful to those reading this thread, the axle stub and circlip part numbers are listed below. You need two of each, they are not side specific.
Axle Stub Part Number: 38415AA110
Circlip Part Number: 805329010
I will let the comparison photos below tell the full story, but just wanted to let anyone out there know that there is in fact a difference.
Early 04 axle stub vs. 06 STi axle... see the step up where it gets larger?
I also made a tutorial on how to change axle seals for anyone looking to do this: http://forums.nasioc.com/forums/show....php?t=1097344