If you want to be competitive in Spec Miata, you need the 4.30:1 Torsen or Mazda Motorsports differential. Most racers simply find a factory Torsen from a 1999-and-up car from a salvage yard and install that.
If you have a 1990 to 1993 car, you need to swap the whole carrier, housing and axle half-shafts, or go with a Mazda comp or viscous differential in the original housing. If you have a 1994 and up car, you can either install the cast iron carrier into the existing aluminum housing, or replace the whole assembly. With 1994 and up cars, you can use the factory axles with the 4.30:1 Torsen.
A rule added to rulebook a few years ago allows you to reinforce the aluminum housing to reduce the likelihood of breakage at the factory “notch” on the passenger side housing extension. That notch was designed to help absorb energy in a crash, but it also can snap during bump-drafting or taking a lot of curbing on a racetrack. It can and does happen.
Mazda Motorsports offers a kit with pre-cut aluminum plates that weld to the differential in the notched area to reinforce it. Part No. 0000-02-5160-MC sells for $35. Call it cheap insurance.
Now, good welders make welding look easy. It’s not. Recognizing my limitations, and knowing that welding cast aluminum can cause annealing, or a softening of the metal, I took my differential and the brace kit to a qualified shop, which welded the braces in place for a cool $125. Better to pay a pro, I figured.
I have done the differential swap both ways: removing and reinstalling the carrier in the aluminum housing while it’s in the car, and pulling the whole housing and swapping the entire assembly. The latter is the easier method, I think, and even if you don’t have a second aluminum housing, it’s easier to pull it from the car and swap the new carrier into the housing on the workbench rather than from under the car.
I have seen books that tell you how to do the swap, but they were all a little light on photos, so I figured we could show as much as possible here in Speed News. Here’s how the job comes together.
These notches in the arms of the differential housing are designed to break to absorb energy in a crash. They also can break in racing conditions, such as bump-drafting or hitting curbing.
Mazda Motorsports sells a brace kit for $35, which welds in place to reinforce the notched area.
The first steps are to remove the rear suspension brace, the exhaust system and drain the differential. 24 mm lower plug, 23 mm upper.
The lower control arms need to be able to extend to full droop, so you have to remove the rear sway bar links from the lower arms. 14 mm socket.
Remove the bolt and nut that hold the knuckle to the upper control arm so you can pull the axle out of the carrier and housing. These are 14 mm.
There likely will be some tension on the upper control arm bolt, and others. You can use a drift pin to punch it through.
The lower shock mount also has to come out to allow the lower control arm to extend. Use a 17 mm socket.
The lower control arm attaches to the knuckle with a long bolt and a nut. Remove the nut and slide the bolt out of one side. I leave the other side in place because it’s one less thing to remove and you can generally get the CV stub out of the differential without removing the lower control arm bolt. This also is a 17 mm.
With the upper and lower control-arm bolts removed, you can pull out the whole half-shaft, knuckle and hub out as one unit.
The driveshaft has to come out, too. It’s a simple matter of four bolts. The factory service manual suggests you mark the shaft and flange to realign them upon reinstallation. 14 mm again.
Some gentle prying with a big screwdriver is all it takes to pop the axles out of the differential.
You remove one axle completely, which lets you leave the other side in. That way there’s less to disassemble.
The power plant frame connects the tail shaft of the transmission to the front of the differential, which has two big vertical bolts holding them in place. They’re 17 mm heads.
Spray some lube on the lower locating washer. You have to pry it loose to get it out so you can pull the PPF away from the differential carrier.
A few gentle taps on a chisel can coax the washer out far enough that you can get a pry bar under it to pry it out. Fun fact: I made this chisel in eighth grade metal shop class.
If you bought your car in a region where they salt the roads, this piece could be really tough to get out. This is a California car, so it wasn’t terrible — as if you needed another reason to hate California.
There’s a cast iron locating bracket bolted to the bottom of the carrier with a 14 mm bolt. Remove the bolt and pry the locating bracket out. The PPF will be much easier to slide off at that point.
This is what the locating bracket looks like out of the car. Note the recess for a locating sleeve on the differential.
The differential has a sleeve in the forward bolt hole. The bolt that goes into that hole has a shoulder that presses that sleeve up into the upper portion of the PPF. That holds everything in place. You either have to slide that sleeve down — difficult if it’s never been apart before — or remove the locating bracket to get the PPF off the differential. Either way works, but you must do one or the other for it to come apart. This is the most difficult part of the removal process, even without excess corrosion.
When the PPF is disconnected, you can remove the nuts that hold the aluminum housing to the subframe. If you have a transmission jack, it’s a good idea to have it in place now so the differential doesn’t fall to the ground when the bolts come out. A regular old floor jack also works. The big nuts are 17 mm.
There are secondary bushings under these caps that press against the aluminum housing. Two 14 mm bolts hold them on. It’s the same setup on both sides.
With the secondary bushings removed, you can see the attachment stud. You can see where the bushings contacted the aluminum housing.
With the assembly fully detached, you can begin to lower it from the car.
This shot provides a good look at the brilliant simplicity of the Miata’s rear suspension.
Now you can lift the new braced 4.30:1 Torsen differential into place.
Guide the sleeved bushings over the studs in the subframe. It helps to have a second set of hands because you kind of have to get both sides on at the same time and jack the assembly up on the trans jack. When it’s in place, put the secondary bushing on and torque the 14 mm nuts to 20 foot-pounds or 240 inch pounds. The big nut torques to 70 foot-pounds.
Tap in the locating bracket and line up the 14 mm bolt. Thread that in and then align the two aft bolt holes.
Reinstall the vertical bolts that attach the PPF to the differential carrier. Remember to install the big locating washer on the front hole. I’m sure there’s a torque spec, but I’ve always just whacked these bolts with an impact gun.
Sometimes the axles pop right in and sometimes they need a little coaxing. It’s not difficult. If it is, you don’t have the splines meshed. If they are meshed properly, turning the axle will turn the pinion.
Once you have all the mounts secured and the suspension bits back in place, be sure you fill the differential with race-quality gear oil. If you forget, you’ll be doing this job again.
Great writeup Brett! Don’t forget to install those secondary bushings back on the diff when you put it back together. We left those off for a season or 2 and had terrible drivetrain stiffness. Those bushings really help stiffen things up!