What are the differential, drive shaft, and ring and pinion interchange options for a Mazda RX-7 FB?

The Mazda FB RX-7 is a great platform for drivetrain parts-bin swapping because it uses Mazda’s ubiquitous 7-inch ring gear. However, before you start buying parts, you need to know exactly what is under the rear of your car, because Mazda split the FB generation into two distinct axle sizes.

Here is the breakdown of your interchange options for the differential, ring and pinion (R&P), and driveshaft.

The 1984 Axle Split

The most important rule for FB rear ends is the year split:

• 1979–1983 models use smaller axles and bearings.
• 1984–1985 models (like the GSL and GSL-SE) use a larger axle housing and larger 26-spline axles ⁷.

Because of the spline count difference, you cannot mix and match axles between these years. Most of the desirable modern upgrades require the 1984+ 26-spline setup.

Ring & Pinion Interchange Options

If you are looking to change your final drive ratio to improve acceleration, you have several direct-fit options for the 7-inch housing. Keep in mind that lower gearing (higher numbers) will make the car significantly faster off the line but will raise your highway cruising RPM.

• Kia Sportage 4x4 (1995–1997): These yield a highly desirable 4.77 ratio. Both the front and rear differentials from the Sportage share this ratio and will work ³.
• Mazda Miata 1.8L (1994–2005): These provide 4.10 or 4.30 ratios and are widely available. Do not buy a differential from a 1990–1993 1.6L Miata; they use a smaller, incompatible ring gear ⁸.
• Mazda B2600 4x4 (1986–1993): The front differential from these trucks contains a 4.44 ratio ³.
• Honda S2000: The ring and pinion from the S2000 is also a confirmed fit for the Mazda 7-inch housing ³.

Differential Carrier (LSD) Swaps

If you want to add a Limited Slip Differential (LSD) or upgrade an old clutch-type unit, you can swap the center section (carrier).

• Miata Torsen: You can drop a 1.8L Miata Torsen carrier into an 84-85 FB ⁴.
• FC RX-7 (1986–1991 Non-Turbo): The LSD center section from a naturally aspirated FC is interchangeable with the FB. You obviously cannot use the FC's housing since it is an independent rear suspension, but the carrier itself works ⁵.
• Early FB to Late FB: If you have an '84–'85 car but source an LSD from an '83 or older FB, you have to swap the side gears inside the differential to accept your larger 26-spline axles ⁴.
• The Kia Sportage Caveat: While you can bolt an entire Kia Sportage third member directly into an 84-85 RX-7, the Kia unit is an open differential. To get the 4.77 gearing and keep your limited slip, you need to pull the R&P off the Kia diff and install them onto your factory Mazda LSD carrier ³.

Driveshaft and Flange Considerations

Do not try to bolt a complete Miata 1.8L third member straight into your RX-7 housing. While the 7-inch housing bolt pattern matches, the driveshaft flange is different and your driveshaft will not bolt up. The correct procedure is to gut both units and transfer the Miata internals (ring, pinion, and carrier) into your original RX-7 housing. When doing this, you must retain the RX-7 specific pinion spacer, crush washer, and bearings ⁶.

Additionally, if you are mixing parts between FB years, note that '83 and older models use a driveshaft flange with a bolt and nut, while '84–'85 models use a through-bolt flange ⁷.

Difficulty Rating

• Moderate: Swapping a complete, pre-assembled third member (pumpkin) into the axle housing. Get the rear of the car up on sturdy jack stands, pull the axles out a few inches, drop the driveshaft, and unbolt the third member.
• Expert: Swapping just the ring and pinion or building a hybrid diff (like putting Miata internals into an FB housing). This requires setting pinion depth, bearing preload, and backlash. I don't have the exact factory clearance measurements for this specific gear setup, so check the factory service manual for the hard numbers on backlash and preload. If you get those tolerances wrong, the differential will whine and eventually destroy itself.