For those of us without STis, our transmission leaves a lot to be desired. Our gears are only sufficient for near stock power levels, and the front and center differentials just plain suck. I've wanted to put together a "decent" transmission for a while now. I say decent because I don't plan on spending 6k+ on upgrading the 5mt. Recent events involving an axle seal leak that went unnoticed for months (huge skidplates require frequent inspection...) have made future plans more of a short term necessity.
Front Differential: OBX. Cheap Chinese torsen variety diff. Good enough
Gears: Stock, maybe RA. I already have late model gears, and I don't plan on doing any significant power mods for a while, so no real need to do much here.
Center Differential: STi 20kgf unit for 5mt. Five times the viscous coupling of the stock center diff.
I received my "like new never used" OBX differential the other day, so here goes. You buy one of these knowing that their are limitations present, and a small amount of work required to get the most out of your purchase; it's 1/3rd the price of anything comparable for a reason. OBX makes diffs for all sorts of cars, so there's already a lot of great info out there on how to deal with the shortcomings present. Read this pdf, and this pdf from this thread for a good idea of what's involved (AKA don't expect a step by step pictorial here EDIT: OK, I lied, check out post 8).
Here's a closer look at what all was involved for me. First up, crappy belleville washers. These washers have a built in curve, and when stacked with opposing curves act as a spring. Junk included, with two of them already squashed permanently flat:
Behold, junk bolts. These were actually of the correct grade, but were beyond rough shape.
The only real rework I had to do involved the oiling holes drilled into both top and bottom halves of the case for the worm gears. The top half was properly countersunk, the bottom not so much. Top:
The holes on the bottom case were obviously drilled after the pockets for the worm gears were milled out, as the blowthrough from drilling was poking into each pocket. Nothing a couple minutes with the dremel can't solve. Don't mind the sloppy countersinking, I was primarily concerned with removing the blowthrough, not with making pretty countersunk holes. You can also just make out that I touched up the knife edges of the worm gear pockets to get rid of some flashing.
Everything was pretty straightforward assembly from here. I stacked the new belleville washers in the following order )()()( for max preload. A little bit of oil on the belleville washers and a steady hand are required to get everything together without the washers moving around. Ready for the top half to go on:
And here it is all bolted together, ready to go into the transmission.
A couple of general notes... I cleaned off whatever oil they used at the factory, and lightly coated all internal parts with 75w90 transmission oil. No need to make a mess, I just didn't want anything getting rusty. Also, if you look closely you will note that this diff has halves that do not completely mate, leaving a ~1mm gap. Don't go all crazy on the bolts expecting the halves to suck together! Finally, you may have noticed that this is a stub axle only design. This means for late model owners like myself, you will need to replace your male axles with those of the stub axle variety. No big deal... new axles, stubs, and new axle seals are required to make the change.
Next up will be acquiring the center diff, and swapping the new diffs into the transmission.