So I'm trying to tackle the job of swapping out the destroyed diff in my new-to-me '90 Miata. I found a used diff, carrier and all, so I'm pulling the old carrier out and putting the new one in. Never done this before, so it's all uncharted territory for me. I'm stuck and need some help from those who have done it before. Here's where I am currently.
Unbolted the axles from the stub axles. My used carrier came with the stub axles in it.
Unbolted the driveshaft at the rear.
I've pretty much stopped there. I see that on the "arms" of each side of the carrier, there is a 17mm bolt/nut that I'm assuming needs to be unbolted, so I did that. Right next to it are a couple of 12mm bolt/nuts that I think are part of a bracket/brace. Tried to remove one side and rounded off one of those bolts (see my rounded off bolt thread).
Am I on the right track? Next steps? Anyone who's done this that could give me a "Replacing Carriers for Dummies" version would be appreciated. I tried to research to see if anyone wrote it up. This is the only one I found. Is it a good write up? http://www.miata.net/hakuna/0012/aindex.htm
mw
HalfDork
2/28/12 7:19 a.m.
You're on the right track. You will need to get those 12mm bolts off. After they are off and the 17mm ones in the centre of the bushings are out, it's just the two bolts holding it to the power plant frame. I had to grind those same 12mm bolt off when I did mine. You new them though, so try not to do that or you'll be looking for brackets. You can run without them, but then you will have an even looser shifter.
Excellent, thanks. I'm going to have to replace those 12mm bolts/nuts, as they have a decent amount of rust. Do you have a good source for those, or did you go to the hardware store?
Some guy wrote a book that includes this 
If you're stopped by those two nuts on each side, you're in trouble when you get to the PPF.
In that case, once you've got the two long bolts off, you'll discover there's a sleeve attaching the PPF to the diff nose. Some cars have a threaded section inside the sleeve, so you put a big bolt in there and pull it down. But not all. So I remove the bolt attaching the spacer to the bottom of the diff, pry the PPF sideways so the spacer can come out, and push the PPF straight up to get it off the sleeve. Bang the sleeve down so it's flush with the frame before installing the new diff.
And use lots of penetrating oil - if your small nuts are rusted that badly, other things will be too.
Another copy of the book written by some guy sold. 
The two nuts on the side didn't stop me really, but after unbolting the axles and driveshaft, I wasn't 100% if I was on the right track for the next steps. I started unbolting the 17mm nuts, then once I stripped the 12mm nut, I figured I'd stop there and make sure I'm doing this right.
OK, so from where I am now....take the two long bolts out of the PPF. Get those 12mm nuts off (the 17s are already off). I'm a bit confused by the sleeve steps...guess it'll make more sense when I see it. Once I figure that out, how do you usually pry the diff sideways? Will the spacer actually pop out completely?
Big prybar to push the PPF sideways. You're not going to be able to move the diff. This might be easiest when the diff is still bolted to the car, by the way. The spacer will come out all the way.
