Stuff needed:
Caution: This procedure was done on a '92 Miata with standard differential. I don't know if it applies to later/earlier models or limited slip diffs. Basically what we're doing here is removing the stub axles from the sides of the differential housing, removing and replacing the seals, then putting everything back together.
Set the hand brake. Leave the transmission in neutral so you'll be able to rotate the driveshaft later.
Jack up the car. The differential housing is a good place to do this, since there are two bosses on it for this purpose. Then support the rear of the vehicle on the jack stands. I put them under the boxes adjacent to the rear tie-down hooks just inside the rear bumper area. I couldn't get the vehicle high enough with my lift to use the side jacking points. Block the front wheels so the car can't go anywhere.
Remove both rear wheels.
Mark the orientation of the axle to the stub axle on the flanges with a scribe or other permanent marker. You want to put them back together in the same orientation.
Next remove the eight 14 mm nuts and lock washers from the axle flanges (four on each side). You may need an extender or a breaker bar to get them loose.
The key to making this a relatively simple process is the removal of the 14 mm nut & bolt at the outer end of each upper rear A-arm (tip provided by Skip Cannon). This is the fastener that attaches the upper A-arm to the wheel/brake assembly upright. (This part is not mentioned in my Miata Enthusiasts Manual - they say to remove the outer axle.) If you have a problem removing the bolt (after you remove the nut!), use a drift punch to drive it out. If it's really stubborn, put your tire jack under the lower suspension member and jack up the upright a little to take some of the load off the upper bolt.
Removing these bolts will allow you to move the outer axles out of the way so you can pry out the stub axles from the differential housing. This is what the big screwdriver or prybar is for. I used the small rectangular boss on each side of the diff housing as a pivot to pry against the bolt heads on the stub axle flange. Do not pry against the diff hsg fins though. They can break. There's a small circular wire clip near the inner end of the stub axle that retains it in the diff. This is what you have to overcome to get it out.
After you get each stub axle released, carefully withdraw it from the diff hsg and set it aside on the side of the car you took it out of on the bolts with the axle part pointing up. This keeps it cleaner and you won't get them mixed up.
Take a flat blade screwdriver and carefully pry out the each old seal. Take care not to scratch the seal bore on the diff housing. It doesn't matter if you damage the old seal taking it out. Note that the replacement seal may not match the original seal exactly. My replacement seal didn't have the outer cup shield that the original one did. This shield was apparently dumped as a cost reduction measure(?).
Clean up the seal bores and the areas around them with a paper towel or rag. Try not to get any dirt or other items inside the diff hsg. Now to install the new seals. But first take a close look at the configuration of the seal. There's a small circular coil spring on the interior of the part that seals to the stub axle. That spring must face the diff housing interior. Then note the short cone-shaped lip that sticks out from the side opposite the spring. That lip must face the road wheel. If you get confused, use the original seal for reference, but keep in mind that the original seal may not look exactly like the new one.
Now smear a little gear oil on the outside diameter of the seal. Place the seal in the seal bore so it's even and not cocked off to one side. Then take a small block of wood and using that, tap the seal gently and evenly into place with a light hammer. Do not whack on the lip that will be sticking out. Place the block on the seal periphery outside that lip and tap the seal GENTLY & EVENLY down into the bore so the entire periphery is flush with the edge of the seal bore. The aforementioned lip will still be above the edge - that's OK.
When the seals are in place, take a little more of the gear oil and smear it on the inside diameter of the seals. This lubricates that surface so the stub axle goes back in easier and doesn't push the seal out of place. Next take the stub axle from the appropriate side and carefully place it back into the diff hsg. It won't go back in all the way. Then take a block of wood and a light hammer and tap the stub axle back into place until it won't come back out (easily). This means the circular wire clip on the stub axle is seated. Repeat for the other side.
Now move the outer axle into position over the stub axle flange. Check for proper orientation using the scribe marks, rotate stub axle as needed, and place the outer axle flange over the stub axle flange bolts. Install the lock washers and nuts and tighten to the proper value (44 ft-lb) using a criss-cross pattern.
Reinstall and tighten the suspension upright bolt and nut. You might need to jack up the suspension a little with the tire jack to get the holes to line up.
Replace & tighten the diff drain plug (in the lower hole). Refill the diff hsg with 0.69 quarts of 90 weight gear oil (until a little overflows the fill hole). Replace & tighten the diff fill plug (in the upper hole). Might as well check the wear on the rear brake pads while you're at it.
Reinstall the road wheels.
Take the car off the jack stands. Take a short test drive and check for leaks.
John Wishart
92 Silver
Member: Peak to Peak Miata Club
Email: john.wishart@compaq.com
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