Some companies could rebuild your differential or just install another LSD helping of opposite lock in mid corner with the power. Not all cars were equipped with Mazda Miata torsen differential. It was originally fitted to a later NA model with 1,8L engine. It is actually a nice upgrade for those considering fitting a turbocharger and making the Miata faster in any aspect. Here is the tutorial from one Miata enthusiast from UK.
To upgrade an NA MX-5 (Miata) diff you must have the following:
1. Replacement diff (obviously)
2. Replacement drive shafts- shortened torsen spec (due to larger size of the torsen replacement)
3. Replacement prop shaft- shortened torsen specific
4. 0.65 litres API GL-5 Differential/gearbox oil
6. 23mm socket (Diff filler plug)
7. 24mm socket (Diff drain plug)
8. 14mm- removal of rear wheels & drive shaft to diff flange
9. Hammer and chisel or equivalent to remove staking from hub nut
10. 29mm socket (remove hub nut)
11. Breaker bar and almost certainly some kind of extension
12. Jack & axle stands
13. Wire brush for cleaning duties
14. Assortment of ‘standard’ spanners and sockets for other bits and piece
15. Replacement exhaust gasket (as you will be splitting the exhaust)
16. Long flat screw driver or equivalent (to remove rubber exhaust mounts)
Mazda Miata differential removal
The night before starting, it might be helpful to spray the areas you need to loosen with some penetrating oil. Once equipment is available you are ready to begin.
Start by simply slackening the hub nut, should have someone stand on the foot brake to prevent wheel rotating.
Remove hub nut
Once those nuts have been loosened, get the wheels off and car in the rear. Then remove the driveshafts from the differential.
Get access to the diff for its removal. This requires taking off the exhaust near the catalyst neutralizer and dropping it off from rubber mounts. Also, don’t forget to unplug the small sensor nearby.
Remove lower arm brace
Disconnect the driveshaft from the differential flange, to prepare it for removal.
To get the drive shafts out you should disconnect the top wishbone from the upright, then drift the splined shafts out allowing them to drop on the ground.
The final stage causing problems is the Mazda power-plant frame (PPF). It is designed to give good throttle response, by rigid connection of differential and gearbox. It is better to loosen it, then try to slide it off the diff and using some string tie it out of the way.
Now you have access to the diff, it is simply a case of disconnecting the propshaft from the diff and removing the old unit. Take note here, the propshaft just slides into the gearbox. Expect gearbox oil to seep out, when you pull it. To avoid mess It is recommended supporting the car at an angle, this helps keep the gearbox oil in the gearbox when you do remove the prop shaft.
The next step is to loose the single 17mm and two 12mm bolts attaching the diff arms to the vehicle. Then, getting underneath and remove the final bit lowering it out by hand.
Rebuilt Mazda Miata differential
To replace differential simply use the instructions above in reverse, this time I would suggest utilising a jack so that you can line up the bolts without having to hold the heavier diff in the air. It would be difficult to mate the PPF with the diff, whilst simultaneously offering up to the car for attacment. Once up get it tightened up quickly, you don’t want to be looking for your 12mm bolts when its in position!
Now would be a good opportunity to replace the diff oil, for peace of mind. I used 0.65 litres of API GL-5. Unscrew both plugs and allow to drain. Replace the lower plug and with the car level fill to the top of the filler (do not over fill). Now replace the top plug, torque to approx 45Nm.
Fix the shorter driveshafts back on. Smear the upright to top wishbone swivel nut in copper grease and reattach. Reattach exhaust and all other things which are lying around on the floor! Put the wheels back on and put the car back on planet earth.
I kept my old driveshaft nut, it is advised to replace however. Now torque up the driveshafts to 260Nm and re-stake them.
The new diff gave the car a slightly different handling characteristic. I have found that turning through a chicane has the effect of locking the outside wheel, this is great, except when you wish to finish the chicace, turning in the other direction. It is impossible without lifting off. Without lifting, the diff remains locked up and is effectively preventing the wheels from taking alternative radii, if this happens to you, you will feel the car’s front wanting to push on forward and the steering wheel becomes incredibly difficult to turn.
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