Mazda 20B Rebuild Tips

The Mazda 20B is a 3 rotor version of the Mazda 13B rotary engine and was manufactured from 89 to 93. This engine was never marketed in the USA and is now being imported for use in high performance cars and airplanes.

There are manuals (Haynes Mazda Repair Manual) and even a video tape by Bruce Turrentine  that describe the procedures for disassembling the 13B but there are none that we aware for the 20B. The purpose of this page will be to describe only the procedures that differs between these 2 models. 

Mazda 20B eccentric shaft separation method, how to separate the 20B eccentric shaft

The primary difference between the 2 models is the 3rd rotor added to the front of the 13B. In order to accomplish this they (Mazda) had to extend the eccentric shaft. and this is the primary difference in the rebuilding process. Instead of separating the housing starting at the rear of the engine per the manual,  you will leave the rear counter balance weight in place. Back out 3 of the bolts in the rear stationary gear housing until they apply pressure on the counter balance ( you could also drive a wedged shaped piece of would between the wieght and the rear housing). This is to keep the shaft from banging around when you are removing the eccentric shaft extension.  Next remove the longer of the torque through bolts and leave in the shorter ones in place, as shown in the photo. All must be loosened a 3/4 turn but only the longer ones removed

See the photo click on it for a larger image
  

 Flip the motor over. You need to remove the front cover per the manual and remove the additional torque bolts that extend rearward through the front housing. When this is done you can tap loose and remove the front rotor side housing, then the rotor housing and remove the rotor. 

You will see the eccentric shaft extension. It is slid down over the shaft and keyed in place, This has to be pulled off the shaft and there several ways that people have come up with to do this and some involve machining a bolt to screw into the extension's lobe and then use a slide hammer to bang it off. However we found that the easiest way is to screw on the front shaft nut all the way. Locate a 4 lbs hammer or a 8 lbs sledge hammer, a 30" crow bar, a piece of leather or cloth and a 12"piece of 1X2. Next place the  piece of 1X2 wood across the rotor side housing in front of the lobe. Place a piece of  the leather or cloth over the end of a crow bar and slip it under the lobe and pry against the  the lobe just moderately. Next  lightly strike the shaft end nut squarely on the top and the the eccentric shaft should separate. 

After the shaft is removed you will be able to see the 12 mm bolt heads that hold in the stationary gear. Leave these alone as they do not need to come out but be careful to not damage the bearing in the stationary gear when you are sliding the side housing up over the eccentric shaft. To do this you need to tape off the threads to protect the bearing. You will need to turn the motor over and remove the torque through bolts that were left in as described above.

After this you follow the manual for the rest of the teardown and just reverse the procedure when you get to the eccentric shaft.

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