I hope nobody minds that I also posted this on M net.

I’ve seen a few write-ups on swapping the 90 – 93 6” diff for a later 7” diff with limited slip. Most topics are covered elsewhere but a couple of things seem to cause some people anguish. Please note that I am not an expert. While I’ve been working on cars for a long time, this is the first time I’ve changed the diff in a Miata.

The first one is removing the spacers from the PPF. The bottom spacer must be pried out but this is fairly easy with 2 small pry bars or flat head screwdrivers. The upper ones are another story. I’ve heard of people prying them out, but I have no idea how they did it, there’s no room up there. Here’s a simpler way:

Remove the 2 bolts securing the diff to the PPF. Remove the lower spacer. Re-install the bolts by hand but do not screw them all the way in. Reach up and feel the top of the upper spacers, as soon as the bolt can be felt protruding from the top of the spacer, stop. From the bottom start hitting the head of the bolt with a large hammer, preferably a dead blow hammer. If the hammer does not have a rubber or plastic face, protect the bolt with a piece of wood. You are going to have to smack it hard but a couple of good whacks should pop the spacer out of the top. Grab the spacer with one hand and unscrew the bolt with the other. The diff should now be able to slide out of the PPF.

Removing the half-shafts appears to be another sticky problem. First spray the axle nuts with PB Blaster or your favorite penetrating lube. To get the axle nut off you MUST un-swag the nut. That is the inner lip of the nut should have been punched down into the slot on the axle, you have to pry it up. Use a punch, chisel and / or flat screwdriver but get the metal up and out of the slot. The nut is swaged to keep it from backing off accidentally and will be a hindrance when you want to take it off on purpose. Next get your impact wrench and the correct size socket. What, you don’t have an impact wrench! Then you should be doing this at a friend’s house who does have one. (No I don’t think one of those battery powered or electric ones will work.) You say all you’re friends live in the same apartment complex and you’re doing this in the parking lot! You’re still going to need the correct size socket but it can be done with a breaker bar, ¾” drive would be best, I’ve heard of people breaking ½” drive bars getting these nuts off! If you’re using an impact wrench ½” drive should be O.K.

Once the nuts are off grab the half-shaft and try to slide it back and forth. If it moves at all you’re a lucky son-of-a-gun, it’s probably going to slide right out once you remove the upper bolt on the upright. If it doesn’t move at all don’t feel bad, you’re in the majority. My favorite way to remove the stuck axle would be to remove the brake caliper and bracket then remove the upper and lower upright bolts and pull the half-shaft out of the diff. Take the whole assembly to a shop with a press and have them press it out. It shouldn’t cost more than a few bucks per axle. If for some reason taking it to a shop is not possible, then you’re going to need a very big hammer and a piece of 2X4 about a foot long. If possible spray the splines with PB Blaster or your favorite penetrating lube, preferably the night before. Unbolt the half-shafts from the diff then remove the upper upright bolt (I actually found it easiest to remove the upper and lower but I’ve been told that you only have to remove the upper). Pull the upright towards you until the half-shaft is clear of the diff, let it hang down and bolt the upright back in place. Have a friend hold the 2X4 on the end of the axle. No, not long ways, the flat side against the axle, then you hit the 2X4 with the hammer. This may take quite a few swings, depending on the size of the hammer. Do not hit the axle directly with the hammer, you will distort it and might not be able get it out of the hub. After a couple of hits, check for movement and adjust your swing accordingly. If it hasn’t moved, hit it harder… Lube the splines with high temperature grease or anti seize when you re-assemble in case you wind up taking it apart again.

If the previous owner of your new half-shafts was an imbecile, as mine was and removed them by hitting the ends directly with the hammer, all is not lost. You could buy a rebuilt half-shaft from your local CV joint repair place. The good news is that it would have new CV’s in it. The bad news is that the last time I checked they were over $100 each! You’re other option is to use a file to shave down the part that has mushroomed out. This took a friend and I about 1 hour with a large file and a small jeweler’s file to clean up the slightly mangled threads. Could you do this with a grinder? Yes but I’m not that steady and you could really mess up the threads with one slip of a grinder. I find that a file gives me more control.

I hope this helps anyone attempting a differential swap.

 

Here's a few tips:

a. make sure the axles isn't siezed in the hub.

It's not smart to do everything first, then spend 3-4 hours with a sledge hammer and utterly destorying it and then having to revert because of (b)

b. make sure the axle bolt is not seized as well. If you happen to have a repalce hub to swap in place because the axle will not seperate, make sure the long bolt can be removed as well so you can actually swap it.

 

The guy who came and bought my diff out of the '92 decided it would be easier to pay me to take the entire diff, rear subframe, half-shafts, hubs, and both control arms, vs. trying to separate everything. :P

Also, when we did those spacers up top, we just removed the PPF from the tranny and tilted the whole thing on one side. Real easy to get that way.

 

My axle nuts were seized. And my axles were seized in the hub. And my 1.8 axle had a seized hub attached. It was fun. Thanks to Ben's dad, we got them all unseized

 

What, no pics? I try to include at least two pics of every step from different angles to cut down on questions... plus, if I ever need to do something again, I'd never take the time to re-read my own writeups cause they take so long... so I cant just look at the pics.

Seriously, nice job... you do me proud

 

we found it easier to move the ppf off the spacers (both upper and lower) by encouraging the ppf with a medium sized pry bar. In fact, my lower spacer would not come out until after the diff was out. Keith Tanner's book had a trick to pull down the upper spacer by using a long bolt (the bolt size info is listed in his book) to thread into the spacer, then pull it down. Like I said, we just used a pry bar to move the ppf off from the top.

As already mentioned, Ben's axles were fun. They had to be beat out and then beat back in. Thread the axle nut on to protect the axle, and use a block of wood to protect the axle nut, then go to town with your BFH. When putting the new axles in, get them started through the hubs, but tighten the axle nuts with an impact to draw the axles into place. That's what we had to do on Ben's car; mine slid on and off without drama.

I can not imagine an impact gun removing the axle nuts. It was a 2 man job with one man holding a big pry bar against 2 wheel studs and the other working a 1/2" breaker bar w/ "cheater" jack handle extension.

Sam, I wanted to take pics, but my hands were too filthy to work the camera.
However, I would encourage anyone who was unsure of what to do to pick up Keith Tanner's miata book. We followed it for the first diff swap (Ben's car) and retained the info for when we did mine, 2 weeks later.

 

Ingersol Rand impact wrench. I forget the model number but I bought it because it could produce over 600 ft/lbs in reverse, something like 550 forward.

Sam, same problem with pictures. The first one would have been of jacking up the car, the rest would have been black smudges.

 

I tried an Ingersol Rand and a Snap On. No dice. No chance. Those things were on rediculously tight.

 

we tried the correct axle removal tool and an impact hammer, no dice. the bottom bolt was also seized so we couldnt remove the hub. The next step is cutting off the hub which we'll do soon.

however, my swap went without a hitch, took a few hours by myself.

then my driveshaft almost fell off last week :sigh:

 

^ 4 ft pole and 1/2 drive FTW.

 

broke the tool. FTL, sorry.

 

for the axle nuts, 3/4 or larger is the only way to go. I broke a 1/2" drive breaker bar doing it that way. Then I bought a Harbor freight breaker 3/4 breaker bar (huge skull cracker) and a socket set to go with and was able to bust the nut. Axles would not come out without pounding, there was no other way. They mushroomed a bit at the tip, but I just ground it down and all was well.

 

Get a huge pry bar. Thread on 2 lug nuts onto the wheel studs, and wedge the pry bar between the 2 studs on one end, and the ground on the other. A second person operates the breaker bar. It will take a considerable amount of force to break the nuts loose. But they will break loose.


(not to scale)

 

Three added tips:

1. If at all possible, make the switch to include the ppf and don't take apart the rear junction of ppf/diff at all.

2. If the axle nuts are on too tight, a Dremel and metal cut off wheel can cut deep enough to crack them off with a cold chisel.

3. And finally, don't pound on the axle very much or you will shorten the life of your bearings. I made up a 1" CRS block with four holes corresponding to the wheel lugs. I knock out the wheel lugs, put four longer bolts through the holes and with nuts on each, press the shaft through.

 

The real issue with what scott was talking about (my axle) was not the nut itself, but the actual axle was siezed. I used a 4 ft pole and one of the axle pushing bolts. The pressure became so great that the actual tool broke before the damn axle would push through. Anyone got a suggestion for that?

 

See 3., above.

 

I'm sticking with cutting off his hub, we cant even remove the bottom bolt because it's so seized as well.

and again, not a matter of removing the nut on this, the actual bolt is bent and rusted and will not release.

 

That didn't work.. used one of those bad boys that locks onto the hub and pushes a bolt into the axle in an effort to push it through.. it destroyed the actual tool. Come to think of it, the sound of metal giving up and the shower of pretty metal flakes was kind of cool

 

Sounds like you may have destroyed the hub and bearings already, so that may be the best course.

 

they were probably already rusted anyways. Luckily he had two hubs with the swap, it's just a matter of getting the old one off.