Differential

1.6 owners beware. The 1.6 rear end is only 6" ring-gear. Open or Viscous LSD.

They are known to fail on stock power delivery. They can blow without warning or last a lifetime. There is really no definitive power level where they will fail. But one way to help prevent it is to show mechanically empathy (i.e. no clutch dumps or drifting). A number of higher powered Miatas have lasted over 20k on them, others longer, some a lot less. Mine (cardriverx) lasted about… 6k miles haha. Get a 7 inch diff, trust me.

Does this mean you should replace it? Up to you, but to play it safe you should plan on it.

But if you do decide to there is still $$$ to be had from ditching the 1.6 rear end to offset costs.


What do you replace it with? The 1.8 rear end of course! Both the Open and Torsen LSD utilize 7" ring-gears, and have been recorded to handle abuse at power levels exceeding 400rwhp (there have been broken torsens at the drag strip at 200whp though). The swap is fairly simple, and it requires only the 1.8 rear assembly, half-shafts & stubs, and driveshaft.

There are various end gears availble from different year Miatas. The 1.6 rear is geared at 4.30:1, so if you're looking for a bit more top-end try to find a 4.10, 3.909, or 3.636 unit. You'll find 1st gear much more easier to handle and a significant drop in rpms in 5th gear.

The 4.30:1 came on:
-1999-2005 5-speed
-1999 auto

The 4.10:1 came on:
-1994-1997 5-speed
-1994-1997 auto
-2000-2005 auto
-2004-2005 6-speed (including MSM)

The 3.909 came on:
-1999-2003 6-speed

The 3.636:1 came on:
-2001+ Austrailan 6-speed

In 2003, Mazda switched from the Torsen to the Tochigi-Fuji (T-F) LSD. The 2003 T-F diffs were prone to failure, so avoid these if you can.

Be advised that the 2004-2005 Mazdaspeed Miata was equipped with a similar 7" ring and pinion, but the differential unit itself is larger. When referring to a "1.8 LSD" the presumption is that the MSM is not included in that category. The MSM also uses larger axles, which are only compatible with the MSM diff.

The MSM limited slip is similar/identical to the limited slip unit found in the Honda S2000.


84-91 RX7 Non-Turbo Clutch Type LSDs can also fit into the 1.8 gear ring if you're looking for a good LSD as well. Clutch-types have proven superior in Drag and Road course situations as the lock under driveshaft torque, whereas a Torsen should prove better in an autox situation as it locks only when one wheel loses torque…so in the middle of corners.

The 84-85 GSL is a 3.909.

The 84-85 GSL-SE is 4.08.


Turbo Mag also reported that these R&P gears will fit

Vehicle Year/Model Final Drive
'94-'02 Kia Sportage 4x4 (front axle) 4.778:1
'99+ NZ market Kia Sportage 4x4 (front axle) 4.625:1
'87-'88 Mazda 4x4 truck (front axle) and Kia NZ Sportage optional 4.44
'79-'85 12a RX-7 3.909
'79-'82 RWD 626 five-speed 3.909
Kia Sportage aftermarket gears are available 4.875:1, 5.125:1, 5.38:1

1.8 open converted to RX7 clutch-type LSD:

Parts:
94-05 1.8 Miata open/Torsen assembly, including ring & pinion
86-88 Rx7 GXL (or '88 Rx7 GTU) differential
94-95 Miata stub shafts OR 86-88 RX7 stub shafts and 94-95 Miata half-shafts OR 96-05 Miata axles (no stubs needed)
RX7 Circlips (fit onto the end of the stubs/axles, miata clips will not fit into Rx7 differential)
94-05 Miata driveshaft
94-05 1.8 Miata aluminum housing OR 86-91 Rx7 NON-TURBO aluminum housing with Miata-style differential bushings

Tip: The Rx7 housing does not have the machined "break-point" that the Miata diff housing has. The Rx7 housing is thus stronger and more desirable. If you're going through all this trouble, use the Rx7 housing if you have one.

Proceedure:
Open the 1.8 miata & pull the open diff. You must use a Miata ring and pinion gear in a Miata housing. The Rx7 pinion gear is significantly longer. Bolt the Miata ring gear to the RX7 LSD. Drop it in, set the backlash (and pinon depth if swapping pinon gear), and reassemble the housing. Swap/install as normal.

*note: The 84-85 LSD will fit, however, the housing is not compatible….unless you want to convert to a solid rear axle
*note: If converting to a RX7 Clutch LSD, simply avoid units from 84-85 models. While the LSD is technically the same, they come out of a solid rear, and there is no provision for the circlips on the stubshafts to lock into the unit, therefore you'll battle leaking axle seals, which isn't fun.
*note: The 89-91 Non-Turbo LSD will also fit, but shy away from these are they are Viscous LSD units and aren't worth the trouble.
*note: Replace the (2) seals on the Diff before reinstallation (MA02-27-238A $5.95)
*note: RX7 Circlips PN is (M005-27-421 $1.95)

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License