Intro
Alright, I just noticed that I can edit old posts. So I'm starting a FAQ post. I will try to keep on this to keep it updated. My plan is basically just a list of useful thread links, specifically for things that are hard to search for (my first thought is 5-lug swap stuff...). If you come across a particularly useful thread or link (does not need to be something posted on the USMB), whether you are the original poster or not, post it up.
I envision this to be a retrofitting-specific FAQ. Focused on using parts from/for one car (usually Subaru model, but not necessarily) on a Subaru. Be it for some sort of upgrade, or just parts availability.
I also think this will lean a little more towards old-gen cars (my specialty was EA82s, so it will start very heavy in that department). New-gen retrofitting stuff is usually pretty well documented on other sites like NASIOC and others.
First place to look, is the Ultimate Subaru Repair Manual. It's current configuration (as another section on the forum) is easier to host and maintain than others, but it does sort of blend in with the rest of the forum. This has been around for about 15 years at this point, so there are more than a few dead links, and outdated information, but it's a great start.
http://www.ultimatesubaru.org/forum/forum/30-ultimate-subaru-repair-manual-the-knowledge-base-of-usmb-complied-posts-and-writeups-to-common-problems-and-projects/
EA to EJ swap manual. This was written 10 years and a couple months ago, so it's a little dated, but hopefully still useful.
Current sources:
http://wordpress.suberdave.com/wp-content/files/EA82/EA2EJ.pdf
This thread is extremely useful for finding people here who have done these swaps. You might find someone who's done the same or similar combination that you are, and you can search for their posts....
http://www.ultimatesubaru.org/forum/topic/80134-ej-swap-whos-who/
Modified flywheel pictures:
http://www.ultimatesubaru.org/forum/topic/79328-todays-project-ej-ea-modified-flywheel-pics/
SPFI EA81 write-up by General Disorder
I think the 5-lug swap is one of the next most important projects that should be in here. It can be hard to search for, and there are so many ways to do it.
Very old post in the USRM: http://www.ultimatesubaru.org/forum/topic/47998-5-lug-conversion-list-plus-tips/
Anything in there that refers to "GC8 Impreza", can actually be any 5x100 EJ-chassis car.
Not to many details on the setup, but many pictures of my first swap: http://www.ultimatesubaru.org/forum/topic/76502-frankenwagon-suspension-build-up/
FYI, XT6 or EJ knuckles use a larger diameter stud on the tie rod end, the EA82 rod ends will not seat correctly. With XT6 knuckles, just use XT6 rod ends. EJ knuckles are shaped differently to change the Ackerman angle, and require a shorter tie rod. Ea81 rod ends work perfectly for this.
Shock/Strut options. After the 5-lug swap, you can run almost any EJ front strut, which gives you a myriad of options for performance options. But, KYB Excel-G is about the only option for the rear that's at all stiffer than stock, and that's just barely. Luckily, "NB" ('98-05) Miatas use a VERY similar shock. They have less travel than the EA82s, but come in 2 different lengths for your ride height preference. Fronts are very close to stock EA82 length, and rears are about 4" shorter. The Miata uses a larger diameter spring, so you will need to use a coilover sleeve kit. Pictures of that install here. This is with the longer front shock, but the upper mount flipped upside-down to lower a bit. I later switch to the standard upper mount, Miata rear shock, with a bit more preload on the spring.
http://www.ultimatesubaru.org/forum/topic/75933-holy-grail-of-ea82-rear-shocks-found/?p=741734
Coilover sleeves
These are pretty handy, as they allow the retrofitting of a very universal 2.5" ID spring to almost any strut, with adjustable pre-load (ride height) for fairly cheap. EA82 rear springs are 2.5" ID already, so these kits fit very easily on the rear. I used a kit that was designed for a "GC" Impreza for many years on my Loyale, both in 4- and 5-lug configuration. I later bought a Ground-Control set that was specifically designed for EJ struts in the front and Miata in the rear. Here's some information on that, cliff notes: I would just order for an Impreza in the future
http://www.ultimatesubaru.org/forum/topic/75933-holy-grail-of-ea82-rear-shocks-found/?p=739855
Complete Coilovers
This is a completely new strut/spring assembly, typically tuned specifically for the chassis for which they were designed. '00-'09 Legacy/Outback have a very similar rear shock design to an EA82, and there are some coilover options for the '05-'09 in particular. I believe WJM (RIP) was the first to do it, but Suberdave was the first to document Tein Flex coilovers in an EA82 body:
http://wordpress.suberdave.com/?page_id=54
'08+ Impreza, '09+ Forester, all Tribecas, all BRZs and '10+ Leg/Outback have a still-different rear suspension design, but uses a very similar rear shock. Nico has used these ('08-'12 STi to be exact):
http://www.ultimatesubaru.org/forum/topic/126559-retro-gl-10-turbo-build/?p=1227259
Others have been used, but it requires putting the EA82 upper mount on the coilover, so the exact fitment will depend on the specific brand and model of coilover, not just the application.
Driveshafts
EA82 4WD 5MT transmissions are all the same length (driveshafts are interchangeable), including XT6.
EA82 4WD 3AT transmissions are the same length as the EA82 5MT
4WD 4EAT transmissions are longer than the 5MT (require a shorter driveshaft).
EJ AWD 5MT transmissions are longer than EA82 5MT, but shorter than 4EAT.
STi 6MT are the same length as the 4EAT.
All EA82 chassis have the same wheelbase, so driveshafts will interchange between body styles (same transmission).
EA81s......What should be mentioned RE: EA81 driveshaft length?
All Subarus use the same slip yoke where they slide into the transmission (Possibly with the exception of the BRZ)
93-07 Impreza 4eat driveshaft just happens to be the perfect length for an ea chassis with an ej awd transmission. The one I used is from a 99, and I had to swap the pinion flange on the 1st gen legacy rear diff because somewhere through the years they changed to a larger bolt pattern than the earlier r160s used. Also had to make carrier bearing mounts, but perfect length, straight out of impreza into ea81 car.
No EA82 or newer Subaru is designed to have serviceable U-joints, they're staked in. But it's been done, there are 2 driveshaft shops here in my town that can rebuild them, so it's fairly common.
https://www.ultimatesubaru.org/forum/topic/168456-driveshaft-ujoint-replacement/
Axles
EA81 front axles at the transmission are all 23 spline
EA82 turbos and XT6s are 25 spline, all other EA82s are 23 spline. 5MT stubs can be swapped, but require splitting the transmission
'93-'94 FWD 5MT Imprezas are 23 spline
All other pre-'05 EJs are 25 spline
After '05, EJs all switch to male splines on the axles that fit into the transmission. Interesting, the stub axles from an earlier 4EAT will pop right into these transmissions, though.
EA81 front axles are shorter (narrower track) than EA82
Early EJ ('90s for sure, probably others) are a bit longer than EA82/XT6 axles, some have reported issues...
GD Imprezas ('02+ Impreza Sedan) have a wider track, and therefore longer axles.
86BratMan posted his recipe for the ball joint reamer he used to modify EA control arms to accept EJ ball joints, and the EJ inner and outer joints that he was able to slip onto the EA81 shafts to use EJ transmission and EJ knuckle with EA81 track width:
http://www.ultimatesubaru.org/forum/topic/162269-converting-4x140-to-5x100/?p=1350309
Transmissions
I'm not going to get into the minute differences between stock replacement transmissions over the years. Here's a really good chart with gear ratios and stuff for EJ transmissions, with transmission code (printed on the bellhousing near the starter):
https://www.rallispec.com/downloads/Transmission ID Chart_Public.pdf
As far as major differences for swapping (mainly talking manual transmissions, as autos introduce a considerable amount of complication with wiring), axle spline counts and driveshaft differences are a major concern. Also, there are a few different transmission mount options...
EA81s had a unique transmission mount situation, and basically any swap will require some sort of custom crossmember
EA82s changed transmission tunnel in '87.5. The mounts on the transmission didn't change, but the crossmember did. Manual transmission XT6 got a 3 piece crossmember like most EJs, the front and rear XT6 crossmembers will bolt up to the center EJ piece, and allow an EJ transmission to bolt in (custom driveshaft still required....but...)
STi 6-speed will fit in a '87.5+ transmission tunnel
http://www.ultimatesubaru.org/forum/topic/97346-will-a-sti-6-speed-fit-in-an-88-gl-yes-pics-inside/
And, of course, rear diff ratio has to match. With the exception of the STi and BRZ, Subarus have all gotten the r160 rear diff. Axle splines and gear ratio are really the only thing that effect interchangeability.
Putting an r180 in an EA82:
http://www.ultimatesubaru.org/forum/topic/111035-goodbuy-r160-hello-r180/
EA82 vs EA81 flywheels:
http://www.ultimatesubaru.org/forum/topic/163014-ea82-flywheel-in-a-ea81/
http://www.ultimatesubaru.org/forum/topic/162986-a-few-questions-on-swapping-to-a-dual-range-tranny/
EJ Modified-MacPherson rear suspension under a Brat
http://www.ultimatesubaru.org/forum/topic/93247-build-thread-off-wrx-pickup/
https://www.ultimatesubaru.org/forum/topic/165430-1982-wrx-brat-swap/
A new creative way to do it:
jdmleggy97 torched the old bearing out, and welded a new face onto the EA82 trailing arm to mount an '00-'04 Outback rear hub/bearing/brake assembly onto the EA82 arm. Now he can use off the shelf rear axles.
https://www.ultimatesubaru.org/forum/topic/168379-writeuptread-loyale-rs-ej-5-lug-hub-fabrication/
If I were to do it, I *might* look into using '08+ Impreza, '09+ Forester, or '10+Leg/OBK parts, as I think that would give more axle options (and therefore diff options). But that's splitting hairs.