Felix's Mazda Rotary Parts Interchange

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:

• - 1974-75
• - 1976-78
• - 1984-85
• - 1986-88.5 NA
• - 1986-88.5 turbo
• - 1988.5-1991 NA
• - 1988.5-1991 turbo
• - 1992- (13B-REW)

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.

Note - Paul Ko has an interesting narrative description of differences between pre-1992 and post-1991 engines at 13BT vs. 13B-REW.

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.

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.

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.

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.

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.