Eccentric Shafts, Stationary Gears & Main Bearings

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Eccentric Shafts

Over the years, a number of eccentric shaft models have been produced and all have proven satisfactory. With the 1986-92 eccentric shafts Mazda introduced a new feature not previously incorporated: a thermal bypass pellet. The thermal bypass pellet serves to limit oil flow to the rotors when the engine is cold, thereby shortening the warm up period to improve emissions. While the risk is small, and not a concern for street engines, there is the possibility that the thermal bypass pellet may leak or completely fail, resulting in inadequate flow to the rotors. In racing use, this inadequate oil cooling can lead to catastrophic failure.

Previously, when building a race engine, we recommended that the 1986-92 stock eccentric shaft be replaced with a 1974-85 type 13B eccentric shaft. By eliminating the thermal bypass pellet, you are eliminating a possible source of failure.

Presently, the RX-8 eccentric shaft has two advantages over prior shafts: It is lighter and it has extra clearance ground into the rear half of the rear main bearing journal. Its only weakness is that it also has the thermal pellet for oil bypass at the front of the shaft. For this reason, we offer an eccentric shaft plug to replace the thermal pellet on all 1986-2006 eccentric shafts.

Prior to installing the eccentric shaft, it is strongly recommended that particular attention be paid to modifying the stock oil jet assemblies, which spray a cooling film into the rotors. The removal and replacement procedure is as follows:

First, the stock oil jets need to be removed. Because Mazda uses Loctite when installing these jets, it is possible that a large screwdriver alone will not be adequate to remove them. If this is the case, heat the head of each jet with a small acetylene torch until the head turns red, then unscrew it. If you work quickly, the shaft will not be heated noticeably.

A check ball and spring assembly is located beneath each jet. For race applications remove and discard these restrictive assemblies. Press a Weber carburetor main air jet (Weber Part No. 77401.200) into the back side of each stock oil jet you have temporarily removed. The jet size will be effectively reduced to 2.00 mm: however, the actual volume of oil flowing through these modified jets will increase, thus providing improved rotor cooling. If the race engine you are building is a high horsepower turbocharged application, you will need to use jets sized around 2.20 mm.

Located at the rear of the eccentric shaft, behind the pilot bearing, is the rear oil galley plug. Beginning in 1983 these plugs were no longer removable. If the eccentric shaft you obtain has a removable plug, even though it may be difficult to remove, the plug will come out through the pilot bearing. It is highly recommended you remove it to allow for a more thorough cleaning of all the oil passages prior to installation of the shaft in your race engine. When you re-install the rear oil galley plug, use two new O-rings to replace the ones you removed, along with new Teflon tape on the threads. The rear oil galley plug should be torqued to 30 foot-pounds. Be sure to adequately grease the pilot bearing and inspect the grease seal.

A weak point of all eccentric shafts is the tendency for the front shaft extension to bend adjacent to the front main bearing journal. This distortion is usually caused by high RPM or some excessive force on the front pulley. (For this reason, caution must be exercised when installing a belt driven supercharger to avoid this distortion.) We suggest both of the following two techniques to minimize this bending problem:

First, increase the torque on the front pulley from the factory specification of 72-87 foot-pounds to about 120 foot-pounds. In doing so, there is a tendency to crush some parts and reduce end play. Do not allow the end play to get below .0015-inch. If necessary, change to a thicker end-play spacer. The stock needle roller thrust bearings have proven to perform satisfactorily with this adjustment.

A second technique to minimize bending is to upgrade the thrust bearings to the type used in the 1992-95 RX-7 and RX-8. These thrust bearings are larger in diameter to better resist the bending load. To accomplish this upgrade you will need to use the following components:

Qty - Mazda Part No. - Description
(1) N390-11-D52 - Thrust Washer
(1) NF01-11-D51 - Thrust Plate
(1) NF01-11-D53 - Counterweight Washer
(2) NF01-11-D54 - Thrust Bearing
(1) N3A8-11-D61 (or RX-8) - Front Counterweight

Additionally, you will need to use one of the following spacers, listed from thinnest to thickest, to set end play:
NF01-11-D55,
NF01-11-D56,
NF01-11-D57,
NF01-11-D58,
NF01-11-D59.

When performing this upgrade, you need to pay particular attention to the front stationary gear. You can either use the 1992-95 RX-7 front stationary gear (Mazda Part No. N3YA-10-E66), manufactured with a larger recess area to accommodate the above components, or you can machine your existing stationary gear.

Should you elect to machine a 1991 or earlier front stationary gear, you will simply need to increase the diameter of the counterbore from the factory diameter of 2.360" (approx.) to 2.520" diameter. Alternatively, Racing Beat offers the "Type II" stationary gears which already have the large recess along with other beneficial features.

If you are replacing your 1991 or earlier front counterweight with the 1992-95 front counterweight, you will need to balance the whole rotating assembly: rotors, rear counterweight and front counterweight.

Stationary Gears and Main Bearings

When building an engine for racing purposes both stationary gears and main bearings deserve special attention. While failures in stationary gears used below 8,500 RPM are very rare, and when they do occur they are typically related to other problems, 8,500 RPM is considered the threshold above which modified stationary gears are required for trouble free performance.

We offer two types of stationary gears for higher RPM operation. Type I Gears, our "traditional" modified front and rear stationary gears, are stock components that have been significantly modified, including induction heat treating of the gear teeth for improved fatigue resistance and greater yield strength, as well as clearancing operations. These gears have proven themselves extensively at 10,000 RPM.

With our Type I "traditional" modified rear stationary gear, we offer the option of a "three window" bearing and additional machining of the internal groove in the stationary gear to allow greatly improved oil delivery. This modification, as well as those mentioned previously, has greatly reduced the incidence of bearing failures.

Type II Gears, a more recent version of the modified front and rear stationary gears we offer, were developed by Racing Beat and are based on 1992-95 stock components. In stock form, both the front and rear stationary gears include: the equivalent of our Racing Beat "three window" bearing with matching oil groove; gas nitriding for increased gear tooth wear resistance; a lock screw to keep the bearing from spinning; and a machining step to accommodate the installation of the large diameter thrust bearings and related components. These Type II front and rear stationary gears are further modified by Racing Beat to include induction heat treating of the gear teeth, for improved fatigue resistance and greater yield strength, and the same clearancing operations as performed on our Type I gears.

The Type II Stationary Gears are compatible with the RX-8 front and rear side housings; however, the rear RX-8 side housing requires particular attention. As delivered from the factory, the rear RX-8 side housing retains the stationary gear oil O-ring in a groove machined into the housing itself. This oil O-ring is compressed by the stationary gear mounting flange to provide oil sealing. When installing our Type II rear gear you have the option of using either the Type II rear gear oil O-ring position or the RX-8 oil O-ring position, or both.

We need to emphasize that regardless of which type stationary gears you use, you must follow all of the Oil System Tips we offer to maintain adequate oil pressure, especially at both high and low RPM.

The main bearing-to-eccentric shaft clearance should be .003" +/- .0005" for operation at less than 8,500 RPM; if the engine will exceed 8,500 RPM, proper clearance is .004" +/- .00025". In addition, bear in mind that as power, rotor weight and/or RPM increase, so should bearing clearance. As any or all of these variables increase, the eccentric shaft bends more, necessitating greater clearance to prevent the shaft from touching the bearings.


Copyright 2007-2014 Racing Beat, Inc ©

Type I Stationary Gears
100x75 12A Front 12A Rear 13B Front 13B Rear
Type II Stationary Gears
100x75 13B Front 13B Rear
Thermal Pellet Bypass Plug
100x75 Bypass Plug
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