So, the SR swap is done, the car is running great, is a hoot to drive, but the lack of a limited slip differential is frustrating, and no matter how much you water them, the money trees in the back yard never seem to grow fast enough to purchase the unobtainium factory limited slip. Even if the factory limited slip differential were to magically show up, there are very few service parts for it available… so what to do?
There are several alternative limited slip carriers available out there, but they are either very expensive, fragile, or locker-style spools—not the best thing for the street. There is an alternative that is worth exploring however! I thought that I would share our take on the Mazda RX-7 rear axle conversion that we are doing in our Datsun Roadsters up here at Spriso Motorsports.
While not as exotic as a four-link conversion, the RX-7 conversions that we are building are bolt in upgrades to the existing roadster rear leaf springs.
In 1984 and 1985, Mazda offered the GSL-SE model which had the upgraded 1.3 liter 13B rotary engine. The GSL-SE had more power, so the Mazda engineers made suspension and brake upgrades to the aging RX-7 chassis to handle the extra power. While Mazda had previously offered a rear-disk brake setup, the GSL-SE had upgraded axles, a new 4 x 4.5 bolt pattern (same as our Datsuns!) vented rear disk brakes, and an upgraded limited clutch type slip differential.
With a little searching at the local wrecking yards or Craigslist, GSL-SE’s can be found, usually with the rear end intact. You will want to pull the entire axle assembly from the car (but you can leave the crazy control arms for a fellow RX-7 enthusiast—we won’t be using them!
Once you have your donor, set up the axle assembly up on the bench, drain the differential fluid and disassemble the axle assembly. When you are done, you should have a bare axle housing that looks like this:
Hmmm. Not so sexy with all those brackets and stuff welded to it… We won’t use any of this bracketry, so out with the plasma cutter or a cut off wheel and let the sparks fly! We won’t have to worry too much about the first 4-5” of the ends of the housing, as these will be removed when we shorten the housing to fit the roadster chassis.
Now it is time to measure and ask yourself what your plans for rear wheels and suspension are. This is a perfect opportunity to allow you to narrow your rear end a bit to fit larger wheels and tires and still get them to fit under the stock roadster sheet metal.
In my case, I wanted to have a housing that was ¼” narrower on each side than the stock roadster wheelbase of 48.5”. I have a 15×7 15mm offset wheel stuffed under my 66’ rear fenders that very occasionally would rub the outside fender. By shortening the housing just a bit, they will now clear without rubbing.
Others wanted to put even more rubber under the stock fenders and we were able to shorten the housing even more to a 47.5” width. By playing with wheel and tire combinations, this car has a 225/40/16 stuffed under the rear wheel wells with on a 7.5” wheel.
Once you have measured once, twice, and three times, cut the end flange assemblies off of the housing. This is easily done with a cut off wheel by cutting through the weld at the end of the casting. If you do it right, you can knock the flange assemblies off with a hammer once the weld has been cut. This leaves the finished edge at the end of the housing that we can now measure from.
Once you have determined how much you are going to shorten the housing, cut the ends off in a band saw (or suitable cutting device). Dress the ends of the cut with a slight chamfer to remove any burrs.
The axle end flange assemblies won’t fit the now-shortened tube, so they will need to be opened up on the lathe so they will fit.
Since we are going to bolt the Mazda axle assembly to the roadster chassis, we need the original spring pads off of the original axle housing. These are easily removed with a cut off wheel. We cleaned and dressed the edges and put them in the bead blast cabinet to get the 40-years of patina off of them.
We set the axle housing up on two blocks on a level surface and put the original roadster spring pads onto a specially built jig that spaces the spring pads correctly. The Mazda axle housing was placed on the spring pads and the pinion angle was set. Datsun set the original pinion angle at 6-degrees and we copied that here.
We triple checked that the housing was sitting centered on the jig, with the same amount of material sticking out both sides. Once satisfied that the housing was centered on the jig, the pinion angle was set and the spring pads were tacked to the Mazda axle housing. Again, the angles and measurements were double checked one last time, and then the spring pads were welded onto the housing.
Our JIG has the location of the original traction bar bracket built into it, so the traction bar was welded to the housing after being ground to suit the Mazda axle housing. The brake T-fitting was cut off of the original roadster housing and welded on as well.
Now that the axle was sitting on the spring pads correctly, the end flange assemblies can be welded on. Since we used the original RX-7 rear brake calipers (with the built in emergency brakes) we welded them on in the same position that they were on from the factory.
To make sure that the end flange assemblies were welded on square, special centering quills were machined which fit inside the flange assemblies. A 1.25” bar of TGP (Turned, Ground and Polished) bar stock was run through the center of the quills to keep everything nice and aligned. It is critical that the flange assemblies are welded on square and straight to the housing to keep everything nice and happy.
The end flange assemblies were TIG welded on and the quills and centering bar were left in the flange assemblies until the assembly had fully cooled so we would be sure that everything is was nice and square.
The axle housing was then sent out for powder coating!
The RX-7 axles were sent up to Dutchman up in Portland, Oregon and were shortened and re-splined to match the Mazda differential. At this time, Dutchman charges $45 per axle.
New brake lines were built and the axle assembly fully built with new calipers and rotors. This one is being installed in Eric’s SR20DET project.
There are a variety of gear ratios and LSD center sections that can be run depending on what you want to do with the car. The Miata Torsen limited slip carrier can be used (which several of us are going to be using in our personal cars) or the factory LSDs can be rebuilt with new clutch packs from Mazdaspeed.
This project would not have come together without the talents of Dave Jolliff, Mike Braaten, and Eric Straw. We have a great crew that is all getting these rear end swaps into their cars this summer. Every one that we are building is a little different to suit the needs of the individual owner, but I thought that I would share the process…