The rearend before cleanup
DISCLAIMER: If you are stupid and/or have no mechanical ability, don't blame me if you get hurt attempting anything I describe. Get help from someone who knows what they are doing. You are on your own, I set this forth for informational purposes only. Just be careful, OK? And for God's sake, get a good set of jack stands and chock the wheels when you jack the car up! Your mileage may vary, close cover before striking, void where prohibited by law.
WHY WOULD I WANT TO DO THIS?
Well, for one, drum brakes on '79 RX-7s are not self adjusting. This is annoying: every so often you have to go under the rear and adjust each shoe. What the hell was Mazda thinking? I have a 1970 Oldsmobile with self-adjusting drums, so I know the technology was available. Whatever. Anyway, discs are, obviously, self adjusting, easier to change pads, are more fade resistant, have more braking area for the same size rotor, and besides, they look cool. All rear disc brake RX-7s were also equipped with a limited slip differential, the benefits of which have been documented endlessly elsewhere (it's a good thing!).
SOUNDS GOOD, WHAT DO I NEED?
Here's the fun part! You thought you were just going to swap a rearend, didn't you? This is the way it works if you are swapping into an 81-85 GS with drums (with a couple caveats), but they changed a couple details between 80 and 81. 79 and 80 RX-7's use course threaded hydraulic fittings on the brake system, while 81-85s have the same thread diameter, but a fine pitch. Result: the rearend itself is a bolt-in, but the brake lines are a no-go. That means you need to change the master cylinder, prop valve, all the hard brake lines, and, of course, the front brakes and lines as well. This is actually a good thing! Why? Well, the 79s have a goofy wedge block caliper mounting system that, frankly, sucks. They fixed that in 80, which uses the same two-pin mounting system as the later models, but there's still another advantage, even if you have an 80: the struts. The strut housings are slightly shorter on the 81-85 cars. This only matters because there are more strut cartridges available for the 81-85, most notably Tokico Illumina adjustables. The strut assembly, like the rearend, is a bolt in (six bolts and a brake line, to be exact). There is one slight advantage to using 84-5 struts: the outer bearing is a larger bearing shared with the GSL-SE and second gens, but I've heard of no problems with the earlier, smaller bearing.
Of course, you're going to need the GSL rearend, calipers, rotors, etc. Don't forget to get the emergency brake cables as well, just the portion from the "frame rail" to the caliper. There should be a clevis and pin secured by a cotter pin just inside the "frame rail" on both sides to detach the rear portion of the e-brake cable from the front part. One final caveat, if you grab an 84-5 rearend, get the driveshaft as well; the driveshaft flange on the rearend is different from that on 79-83 models. Or, you can swap the drive flange from the old rearend to the new to mate with your existing driveshaft. Good time to replace the pinion seal, too. This can be done without having to reset the pinion preload, just retorque the pinion nut to 90-110 lb.ft. when you put it back on, and it should be fine (so says the factory manual). I had no problems.
So, the final list:
GSL disc-brake LSD rearend, with e-brake cables, rotors, calipers, etc.
Driveshaft, if 84-5 rearend.
All hard and flexible brake lines (good time to upgrade to stainless steel braided).
Master Cylinder, with proportioning valve.
Strut assembly, with rotors, calipers, etc.
All those little clips and things, just so you have spares.
The toughest part of all of this is getting the hard brake lines loose from the flex lines. After twenty years, those soft fittings stick pretty tight, and it's a job getting them loose without mutilating them. Forget the flare nut wrenches; they'll destroy the flare nuts just as well as regular wrenches. I used copious amounts of Liquid Wrench, and a set of Channellocks. The Channellocks allow you tho get a good grip, and added leverage. Worked for me, even after I made a couple of the fittings round.
THAT'S A LOT OF STUFF, AND THAT REAREND WAS BLASTED HEAVY, BUT I GOT IT ALL. NOW WHAT?
If you don't have one, get a service manual. Hayne's or Clymer's will work, Factory manual's even better. I like to have several different publishers; sometimes one will cover something better than the other. This is not going to be a comprehensive step-by-step, that's what the manual is for. Besides, if you got this far, you already know how to get the rearend and struts out, don't you?
You may want to clean and pack or replace bearing, seals, etc., resurfacing or replacing rotors and brake pads, replace third member gaskets, change rearend lube. etc. on your replacement stuff. Better put new cartridges in those struts, too, while they're out. Easier to do it all now, and you'll know when it was last done.
From this point, there's nothing really difficult, just remove the original struts, rearend, master cylinder/prop valve, and all brake lines. Actually the most difficult part for me was breaking loose the fittings on the brake lines. Good idea here is to soak the fittings with Liquid Wrench once or twice a day for about a week before you try to crack 'em. The hard lines take a little finagling to get out without tweaking them too hard, but it can be done. Another tip: jack the car up as high as you can at both ends and put it on jack stands; you want plenty of room underneath to work, and you'll need the jack to get the rearend up and down.
OK, now put it all back, only use the 81-85 stuff. don't forget to connect the emergency brake cables. Since you're working on the front end anyway, why don't you replace all the bushings with polyurethane? Check tie rod ends, idler arm and ball joints while you're under there; it's easiest to replace them now, while it's all apart. Double check all fasteners, flush and bleed, put her back on the ground, and *carefully* test. Troubleshoot as needed. Get an alignment. Enjoy being able to *stop*! May even handle better if you did a lot of front end stuff as well. Have a beer.