Felix's Mazda Rotary Parts Interchange


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US Ring & Pinion Ratios
616'714.111automatic only
61x, 1x00All3.700boinger sedans
RX-7 12A'79-'853.909& 6-speed Miata
RX-7 12A'843.933part year
GSL-SE'844.077thru 11/84
GSL-SE'84-54.08312/84 up
B-xx00 Pickup'87-84.4444WD M/T
Miata'94-74.100& 99+ automatic
Miata'99-4.300M/T only

1600, 1800, 616, 618, RX-2, RX-3SP, SA & FB RX-7, RWD 626 and '86-up Mazda "B" series pickup truck third members (also aka pig or pumpkin) are interchangable. 1984-up models use larger axles, so the side gears in the differential are correspondingly larger-splined and must be swapped for correct gears if substituting into an earlier housing with smaller axles. Pre-SP RX-3s can use the later third members if an SP driveshaft is substituted along with it. Some pickups had 4.44 ratio gears, but never in conjunction with limited slip. The Mazda part number for the 4.44 pickup ring & pinion gearset is M054-27-110A. As the chart shows, '94-up Miata gearsets are also interchangable. See Randy Stocker's Miata Drivetrain Interchange page for more information. See Driveshafts for related differential flange issues. See Rear Axles for additional related issues.



Mazda rotary distributors from 1981-85 are an integrated electronic type suitable for use in any Mazda rotary for model years 1974-91. The OEM crank angle sensor is preferable for use in the original car, but for swaps in which the OEM computer is not part of the plan, the 1981-85 type works fine. While the mechanical advance curve in GSL-SE distributors is usually better suited for 13B applications, and 12A distributors can be recurved to 13B spec, this is not required for basic function. 1980 distributors are also electronic, but they are multipart and less reliable than the integrated later models. Tim Stiles & Kevin Wright have provided wiring diagrams for the 1981-85 distributors.


RX-3:Because later RX-3s use the same type differential center section (pig) as RX-2s and RX-7s, use of the longer later style RX-3 shaft facilitates straight drop-in of the pigs with shorter gears and limited slip found in RX-7s.

RX-7: Except for 4-speed automatics, which used a 5 cm shorter shaft, all 1st generation RX-7's used the same length driveshaft. 1979-1982 shafts have different differential companion flanges than 1983-1985 shafts. 1979-1982 shafts have normal U-joints, while 1983-1985 shafts have non-replaceable U-joints. By using the matching differential flange, fasteners, and seal, either group's shaft can be substituted to the other. Most driveshaft rebuilders are not competently capable of replacing the non-replaceable U-joints, which means some are, but they are hard to find. Replacement 1983-1985 shafts with replaceable U-joints are available from Mazdatrix. Good driveshaft shops can build replacements from scratch if you provide the old one for dimensional comparision. All NA rotary transmission yokes are the same spline and length. RX-2 and RX-3 differential flanges and seals are the same as 1979-1982 RX-7. The later flanges use through bolts with nuts. The early ones use cap screws that screw into the flange. Those threads help one to distinguish between the two types.

RX-7 Turbo: Transmissions for production turbo engines use a larger output shaft diameter and thus a larger driveshaft yoke. The length of a turbo driveshaft is the same as the length of the NA driveshaft for the same generation cars.


Automatic Engine in Stick Car - In simplest terms, just add pilot bearing & seal and appropriate flywheel. Technically, there can be balancing difference issues, so to use a stock flywheel, it needs to be selected to match your year engine's internal rotating parts. A nice way around this is to leave the automatic counterweight on the engine, and bolt on a light flywheel from Mazdatrix or Racing Beat or elsewhere. They will reduce the flywheel price when you inform them that you already have the counterweight. That will provide a performance boost and weight savings. In most cases, there are minor differences in emissions controls between auto and stick engines. Removing such parts from a new auto engine and installing those from your old stick engine is generally a good way to avoid quirks that can result from mismatch.

Transplanting Engines of Different Years The difficulty can range anywhere from not difficult at all to vexing. Differences that by themselves prevent interchange have been surprisingly small over a vast period of time. A large part of this subject is covered under flywheels, transmissions, replacing a first generation RX-7 13B with a second generation RX-7 13B, and replacing a 12A with a 13B. The remaining issues break down to essentially two broad categories: 1- physical fit, and 2- maintaining the emissions control system in a compliant state. This section is intended to point out issues not addressed elsewhere on this site.

12A to 12A - Physically, all from model years 1974 - 1985 are fully interchangable if used with headers and an appropriately selected intake manifold.

Mazda began using catalytic converters instead of thermal reactors with the 1981 models. This resulted in a routing change for air from the air pump. For the emissions control system to remain fully functional, 1981-85 engines cannot be substituted for 1974-80 engines, or vice versa.

1976-80 intake manifolds used on 1981-85 engines leave the air port in the center housing open. In the absence of some creative means to plug the port, it functionally becomes an exhaust port venting to atmosphere.

The 1974-75 engines had more agressive port timing. Substituting one of these models into a later car might result in failing emissions tests in spite of all controls being in place and functional.

Subject to limitations imposed by the above two emissions control groupings, emissions compliance is maintained by changing only the bare block and retaining all the other components that came with the car.

13B to 13B - In a broad sense, any 13B can be substituted for any other 13B. Swaps are only simple within the following groups:

The obstacles come when trying to interchange variants. With the 1986 models, the engine mounts were moved from the front, where all previous variants had them, to the center of the engine. Though this resulted in a change in sump configuration, the bolt pattern for the oil pan remained the same, permitting early pans to be used on later engines, allowing their use in earlier cars by converting the mount point back to the front of the engine. With the REW engines, the mounts were moved to the rear housing. The intakes are of three different, non-interchangable, basic types: 1- 6-port; 2- pre-turbo 4-port;, & 3- turbo 4-port.

Engine parts

Group 1
  • 10A
    • Cosmo Sport
    • R-100
    • RX-3 (non-US only)
  • 12A
    • RX-2
    • RX-3
  • Basic features
    • "4-port" designs - 2 intake ports per rotor
    • 60 mm wide rotors - 10A
    • 70 mm wide rotors - 12A
    • 6 mm wide one-piece carbon-aluminum apex seals
    • Long, flat depressions in rotors (12A)
    • 12 side seals per rotor, 1 mm wide
    • Compression seals fitted into rotor housings
    • 4-bbl downdraft carburetors
    • Two separate distributors
    • Weak low-RPM torque
    • Engine mounts in chassis via front cover

  • Other characteristics
    • Top mounted starters (most models)
    • Highly succeptible to compression loss due to overheating
    • Commonly hard to start after seal wear accumulates or rebuilding with used parts
    • Uses thermal reactor system for emissions control
    • Post-Cosmo side housings are interchangeable between 10A & 12A
    • Complete rotating internals can be retrofitted from Group 2
Group 2
  • 12A
    • RX-2/Capella
    • RX-3/Savanna
    • RX-4/Luce (non-US only)
    • RX-7 - "6-port" available in Japan only beginning 1982
  • 13B
    • RX-4/Luce
    • REPU
    • RX-5/Cosmo
    • RX-7 "6-port" only
  • Basic features
    • "4-port" designs - 2 intake ports per rotor
    • "6-port" designs - 3 intake ports per rotor
    • 70 mm wide rotors - 12A
    • 80 mm wide rotors - 13B
    • 3 mm wide two-piece steel apex seals
    • Deeper, shorter depressions in rotors
    • 6 side seals per rotor, 1 mm wide
    • Compression seals fitted into rotor housings
    • Same bearings used as Group 1
    • Slightly changed intake & exhaust port to stud alignment
    • 4-bbl downdraft carburetors (4-port models)
    • Electronic Gasoline Injection (6-port models), 2 injectors
    • Single distributor
    • Improved low-RPM torque - 4-port models
    • Vastly improved low-RPM torque - 6-port models
    • Improved cooling system design

  • Other characteristics
    • Bottom mounted starters in production vehicles
    • Far less succeptible to compression loss due to overheating
    • Easier starting than Group 1
    • Uses thermal reactor system for emissions control (earlier models)
    • Uses catalytic converter system for emissions control (most later models)
    • All 4-port side housings are interchangeable between 12A & 13B
    • Engine mounts in chassis via front cover, which are all physically interchangeable. 6-port covers have a distinctive mount for an oil injection pump with 4 instead of 2 outputs.
    • 4-port 13B engines can be built using a mixture of any 12A side housings with 6-port intended rotors & rotor housings and intake & exhaust systems from earlier models or aftermarket
    • Considerable casting thickness provides ample opportunity for significant porting gains
    • Front covers interchangable between 12A & 13B, but oil metering pump must match front cover. 6-port engines use a metering pump with four outputs that inject oil into intake stream and into rotor housings. All others have two that inject into intake only.
Group 3
  • 13B "6-port" NA RX-7
  • 13B "4-port" Turbo 2 RX-7
  • 20B "4-port" Cosmo
  • Basic features
    • "4-port" designs - 2 intake ports per rotor
    • "6-port" designs - 3 intake ports per rotor
    • 80 mm wide rotors
    • 2 mm wide three-piece steel apex seals
    • Deeper, shorter depressions in rotors, like Group 2
    • 6 side seals per rotor, .7 mm wide
    • Compression seals fitted into side housings
    • Same bearings used as Group 1 & 2
    • Electronic Gasoline Injection, 2 injectors per rotor
    • Distributorless electronic ignition timing control

  • Other characteristic changes
    • Engine mounts in chassis via center housing
    • Reduced casting thickness provides lesser opportunity for significant porting gains

  • Other
    • Suitable for transplanting into earlier rotary cars with a front cover and oil pan swap. Using anything other than a GSL-SE front cover with its 4-output oil metering pump unnecessarily complicates the job if using the Group 3 OEM intake system. Exhaust door actuator on turbo manifold will not clear an unmodified 1 gen engine mount frame.
Group 4
  • 13B-REW RX-7
    • US/Canada
    • Elsewhere
  • Basic features
    • "4-port" design - 2 intake ports per rotor
    • 80 mm wide rotors
    • 2 mm wide three-piece steel apex seals
    • Deeper, shorter depressions in rotors, like Group 2 & 3
    • 6 side seals per rotor, .7 mm wide
    • Compression seals fitted into side housings
    • Electronic Gasoline Injection, 2 injectors per rotor
    • Distributorless electronic ignition timing control

  • Other characteristic changes
    • Larger rotor and main bearings than previous versions
    • Highly sophisticated (complicated) electronic engine management system
    • Design detail changes make transplants into earlier rotary cars difficult
    • Two turbochargers
Note - Paul Ko has an interesting narrative description of differences between pre-1992 and post-1991 engines at 13BT vs. 13B-REW.

Flywheel, Clutch & Starter (U.S.)

Starter - All M/T Mazda rotary starters from 1974 through 1986 plus NA M/T starters from 1987 through 1991 can be interchanged due to identical flywheel ring gear diameter and tooth count.

Flywheel & Clutch - All Mazda rotary flywheels can physically bolt to any Mazda rotary engine.

Interchange issues relate to:
  1. Ring gear matching to starter & bellhousing
  2. Matching imbalance to the engine
    • 12A 1974-1982
    • 12A 1983-1985
    • 13B 1974-1985
    • 13B 1986-1988 NA
    • 13B 1987-1988 Turbo
    • 13B 1988.5-1991 NA
    • 13B 1988.5-up Turbo
  3. Clutch size
    • 215 mm (1974-1982) except REPU
    • REPU
    • 225 mm (1983 - 1991) NA only
    • Turbo
About light flywheels.
Stock flywheeels have an imbalance built into them to match the counterweight under the engine's front cover and the weight of the rotors and eccentric shaft. Mazdatrix has a photographic flywheel identification page you may need if working with used parts or troubleshooting vibration. Stock flywheels are attached to the eccentric shaft, which is tapered on the end like is typical with small engines, with a single nut fit by a 54 mm socket. Aftermarket flywheels are supposed to be in perfect balance and are affixed with six bolts. An adapter called an automatic transmission (A/T for short) counterweight is first affixed to the eccentric shaft with the 54 mm nut, and it contains the required imbalance and a flat flange with the six bolt holes required by the replacement flywheel. This means that replacements are primarily of two basic types:
  1. Those designed for both 215 & 225 mm clutches
  2. Those designed for Turbo clutches
which are available in
  1. solid steel, and
  2. aluminum with a steel face for the clutch facing.

Additionally, the REPU clutch and flywheel is an entirely separate beast. If you intend to replace an REPU flywheel with aftermarket, you must choose one of the other 13B types and use a clutch appropriate for your chosen flywheel.

Additional part numbers listed in catalogs and such web sites as Mazdatrix are necessary only because their "flywheels" are packaged with the A/T counterweight suitable for the intended application.

Hydraulic Threads 2006/12/28

Mazda used to use a coarse thread pitch, 1.25 mm, on brake and clutch hydraulic line fittings. Cars prior to the RX-7 used these coarse threads, and the RX-7 continued them through the end of the North American 1980 model year. 1981 and later North American RX-7s use the same size fittings, but with a fine thread pitch, 1.0 mm. This changeover typically causes frustration when not planned for in swapping later rear-ends or struts into 1979-80 cars, but can also happen via a breakdown in the new or rebuilt brake parts supply chain.

Rear Axles

Early Rotaries - R-100, RX-2, RX-3, RX-4, Cosmo, & REPU axles remained the same throughout production, but, IIRC, each model uses different parts, so interchange among the models is 0%, while interchange among years is probably 100%. Right & left RX-2 are equal length. RX-3 right side axle is shorter than left side.

RX-7, 1st Gen, '79-'83 - Except for splines, which match those of RX-2 and RX-3, RX-7 axles are entirely different from earlier axles. The right side is shorter. All axles and wheel bearings are interchangable, but backing plates may need to be changed, as they are entirely different between disk brake and drum brake cars.

RX-7, 1st Gen, '84-5 - Axle diameter and wheel bearing size was increased. Because of the wheel bearing change, large axles can only be fitted to housings designed for the larger bearings. Due to the difference in lug pattern and brakes, 12A and 13B axles cannot be swapped except complete with brakes and wheels. It is far easier to change the whole assembly. As with earlier models, backing plates must match the brakes to interchange axles.

Axle Housings & Assemblies, RX-7, 1st Gen - All are interchangable among all years as complete assemblies. The bearing size change in '84 limits bare housing interchangability to pre-'84 and post-'83. Further, backing plates attach with four bolts on drum brake cars and 3 bolts on disk brake cars, further limiting bare housing interchangability. All '84-5 disk brake axle housings carry the same FA58-26-020 part number. For swapping a later assembly into a pre-'81 model, or vice versa, see Hydraulic Threads.

See also Differential.

Transmission - Bolt-Pattern

All Mazda rotaries from US model year 1974 on are equipped with the same bolt pattern. All such transmissions can be bolted to all such engines. The large-clutch flywheels used on turbo models will prevent NA transmissions from being installed, but with no flywheel installed, an NA transmission will bolt up.

Transmission - Length

All North American spec Mazda rotary manual transmissions from 1974 through 1991 are the same length (~32") from the tip of the input shaft to the tip of the output shaft, with the possible exception of the 1974 RX-2, which may be around 40 mm shorter. Bellhousing and input shaft were shortened for 1993-up models.

Transmission - Suitability

Group 1

1974 through RX-7 SA/Series 1

Simple functional equivalent bolt-in:

  • RX-3 1976-8 5-speed
  • RX-4 1976-8 5-speed
  • REPU 1976-8 5-speed
  • Cosmo 1976-8 5-speed
  • RX-7 1979-80 5-speed, 4-speed
  1. These 4-speeds have the same length, bolt pattern, and shifter location as the later 5-speeds, but the mount pad is slightly forward.
    • RX-2 1974 (maybe, see below)
    • RX-3 1974-5
    • RX-4 1974-5
    • REPU 1974-5
  2. Group 2 boxes can be substituted into cars in this group if console modifications are made.
  3. 1974 RX-2 transmissions may be identical to 1974-5 RX-3 & RX-4s. If not, the difference would be a shorter overall length. Pre-1974 RX-2 transmissions were shorter than the earlier RX-3 transmissions, which were the same length as this group.
  4. Late production 1980 boxes can have the tailhousing and entire shift mechanism from Group 2 or 3 boxes installed for use in Group 2 or 3 cars. The 1980 boxes that have this compatibility are distinguished by their use roll pins instead of cap screws to mount shift forks and fingers to their rails.
Group 2

RX-7 FB/Series 2 & 3

Simple functional equivalent bolt-in

  • 1981-3 5-speed
  • 1984-5 12A 5-speed
  • 1984-5 13B 5-speed
  1. 1981-3 ratios and other details are all identical
  2. Several 1984-5 internal basics, including ratios, are different from earlier boxes. SE boxes used two different 5th gear ratios, both of which were taller than 12A boxes. There were also a few running changes in internal design details.
  3. SE boxes are easily distinguishable externally by two large air vents cast into both sides of the bell housing.
  4. These boxes can have the shift mechanism from Group 3 boxes installed for use in Group 3 cars. The expedient way to do this is a complete tailhousing swap.
  5. These boxes can have the tailhousing and complete shift mechanism from late 1980 boxes installed for use in Group 1 cars.
Group 3

RX-7 FC/Series 4 & 5 NA

Simple functional equivalent bolt-in

  • 1986-91 5-speed
  1. Basic design of these boxes is identical to the last Group 2 boxes.
  2. 1-4 ratios are identical for all years, but different from the latest Group 2 boxes.
  3. 5ths differed by model and year.
  4. These boxes can have the shift mechanism from Group 2 boxes installed for use in Group 2 cars. The expedient way to do this is a complete tailhousing swap.
Group 4

RX-7 FC/Series 4 & 5 Turbo

Simple functional equivalent bolt-in

  • 1987-91 5-speed
  1. These boxes are derived from the much stronger pre-RX-7 rotary boxes. Synchronizer design was significantly improved, and the output shaft size was increased, requiring a larger driveshaft yoke.
  2. Turbo models use larger clutches and flywheels and a different starter than NA models.
  3. Bellhousings from the pre-1979 boxes can be substituted for use with NA starters, flywheels and clutches, partially easing their use in 1st gen cars. Sales volume from 1974-78 was sufficient that finding a used such transmission to cannibalize should not require heroic talent. The other obstacle to their use in 1st gen cars is the shifter location much farther rearward. Solving this latter problem requires some clever shifter reengineering (possibly using a portion of the original transmission's shift mechanism) or a hatchet job on the console.
  4. Ford used a variant of this transmission in some of its products in the 1980s.
Footnote FAQ

The 1971-1985 Racing Beat Technical Manual & Catalog regarding 1984-1985 5th gear ratios on page 77 says:

1984 12A, .758; 1984 13B & 1985 12A, .807; and 1985 13B, .711

Those ratios match mispublished official Mazda information. However, the following is what really happened:

  1. All 1984-1985 12A cars were equipped with .807.
  2. The initial run of SE transmissions was .758.
  3. Mazda made a mid-year change during 1984 to .711 that carried through the 1985 models.

Automatic to Stick Conversion

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