Miata Differential Rebuild & Assembly Instructions
Let's build a diff! Whether you're doing a upgrade to a Torsen limited slip, or just rebuilding your open diff to make it fresh and happy- the instructions and steps are all the same. Take your time, don't skip steps, and keep your work area clean.
In this document we will use photos from both 1.6 and 1.8 Differentials. Conceptually they are the same with minor differences. All fasteners use the metric system.
Tools You’ll Need:
· Dial indicator
· 5–6-inch Micrometer -(1.6 diff only)
· 7–8-inch Micrometer - (1.8 diff only)
· Torque Wrench (Up to 130 ft-lbs)
· Deflection Beam Torque Wrench (Up to 20 in-lbs)
· Punch
· Hammer
· Diff Stand or Holding Device
· Hydraulic Press
· Bearing Separator
· Bearing Press Tool Kit
Supplies You’ll Need
· RTV
· Red Thread Locker
· Blue Thread Locker
· Shop Towels
· Motor/Gear Oil
Disassembly
After opening the case this is what you should see. (1.8 Diff shown)
Before you remove the caps it's very important that they don't get mixed up. they need to go back onto the same side. Use a punch and mark on one side as shown.
After you have your caps marked you can start disassembling. Remove the retaining tabs for the adjusting screw. Take off the caps. If you are reusing bearings, it is important that you mark which ones went where as well.
Now you can lift the differential carrier out from the housing.
To take the ring gear off, you will need to remove the bolts from the backside. If the ring gear is stuck, just use a rubber mallet and hit it from side to side wiggling it off.
Now you can go ahead and remove the nut and flange from the pinion. That will give you access to the pinion seal which you can also remove.
Beware of the washer that is on top of the bearing. Be sure to remove this and save it for later as it is very important.
Once those are off you can start the whacking process. Removing the pinion is going to require some force. If you can get it out with just a rubber mallet then great. I’ve found that a punch on the end of the pinion and a metal hammer is a often required to push it out. 
Now that your pinion is out you can set it aside and press out the races. Use a chisel/punch and hammer to tap these out. Just work the races side to side and they’ll come out eventually. This method works for both pinion bearing races.
Pressing Bearings
This step is relatively easy as long as you take your time to line everything up and be careful not to break anything. First we will start with the pinion. There is only one bearing to press on and off with the press on the pinion. You should fit your Bearing Separator tool in between the gear and the bearing. Make sure you have a hand underneath to catch the pinion. Once its out there should be a washer that was underneath the bearing. DON’T LOSE IT.
You can go ahead and follow the same process for the carrier. Insert your Bearing Separator tool in between the carrier and the bearing. Be sure you are pushing on the inside piece of the carrier and not the outside bearing race. Perform this action on both sides.
Pressing everything back together is even easier. Clean all your parts thoroughly before assembling. For pressing on the carrier, I like to take an old bearing and repurpose it for a pressing tool. Now you can press on both carrier bearings.
For the pinion, make sure you have that washer on before you press on the bearing because it is very difficult to remove the bearing without damaging it. Using a section of tubing, press the bearing on the the shaft until it seats.
Assembly
For pressing the races into the housing, you need a properly sized bearing press tool and a mallet/hammer. When pressing the races in don’t let the race get crooked because it will get stuck. Press them in till they bottom out.
Insert the pinion into the housing and slide the crush washer and race on the backside of the pinion. Next slide on your washer and install the pinion seal. Slide on the flange and begin threading on the new lock nut. Lubricate your threads and bearings with oil.
Pinion Nut Torque Spec: 86-130 ft-lbs
You should not feel any play in the pinion shaft.
Pinion Drag: 10-15 in-lbs
Slowly work your way up in pinion nut torque till you get the proper drag. make small corrections. 1/8 turn then check the drag. Measure in both directions. The drag is more important than the torque number.
To install your ring gear back on your carrier. Line up the bolt holes when you slide it on. Then tighten the bolts in a star pattern like you would do for wheel lug nuts. Work your way up in torque in 3 steps. (20-40-61 ft-lbs Red Threadlocker)
You are now able to install your carrier and bearing races into the housing. Install the adjusting screws and caps. You should be able to press the caps all the way down by hand. If you can’t it’s because the adjusting screws aren’t threaded in correctly. Torque your caps down. (38 ft-lbs Blue Threadlocker)
Setting Backlash
Lock the pinion shaft is a solid position so it can’t move. Screw in the adjusting screws so the ring gear is up to the pinion. They shouldn’t be super tight but they also shouldn’t be loose enough that you can turn them with you fingers. Put on your dial indicator so that it is 90 degrees from the ring gear teeth. This is so that you get a proper backlash measurement. Rock the ring gear back and forth to measure the amount of backlash you have. (0.0035-0.0043 in)
If your backlash number is too small your ring gear is to close to the pinion gear. If your backlash number is too large your ring gear is to far from the pinion gear. Adjusting the screws in will move your carrier in and out. When adjusting the screw you should turn them both the same amount. To turn the screws it is easiest to use a mallet and a punch.
Bearing Preload
Your bearing preload measurement tells you how far out the caps have been splayed from your adjusting screws. If your measurement is too large then your adjusting screws are too tight. If the measurement is too small your screws are too loose. The measurement spots on a 1.6 and a 1.8 are in different spots but they are conceptually the same. They will me machined flat spots on each cap and you will measure across the housing.
1.6 Diff Preload (5.9105-5.913 in)
1.8 Diff Preload (7.3004-7.3031 in)
Both sides of the caps should measure within spec.
When all your measurements are within spec you are good to put in your retaining tabs. (19-ft-lbs Red Thread-locker)
For the aluminum half of the case. The only work that needs to be done is replacing seals and cleaning the sealing surfaces. For pushing out seals put your bearing press tool inside the case and push the seal out. I run a socket extension through the opposite hole and tap from the top. While the seals are out you can clean the surface where they sit with some scotch-brite. Pressing the seals in is pretty straightforward. Line up the seal and press it in while making sure not to get it cocked to one side. On the 1.8 diff there is a shelf for the seal to stop on. Be sure to press it all the way in so it is sitting on the shelf. Tilting the press tool to one side allows you to be able to press it all the way in. I clean the housing sealing surface with scotch-brite and a razor blade.
Differential Specs:
Pinion Nut Torque: 86-130 ft-lbs
Pinion Drag: 10-15 in-lbs
Ring Gear Bolts: 61 ft-lbs (In 3 Steps)
Bearing Caps: 38 ft-lbs
Retaining Tabs: 19 ft-lbs
Ring Gear Backlash: 0.0035-0.0043 in
1.6 Diff Preload: 5.9105-5.913 in
1.8 Diff Preload: 7.3004-7.3031 in