All rotor bearings, stock or Mazda Factory Race (MFR), are made from a flat piece of steel which is stamped, rolled, and hooked together at the joint with a "puzzle lock" design. This bearing is then coated with copper and plated with about .0015" of Babbitt bearing material. The inside surface of the puzzle lock joint area is ground off to eliminate high spots, usually resulting in a portion of the copper coating being exposed. This appears, to the untrained eye, to be a worn spot on the bearing, but is in fact normal (See Photo).

Rotor bearings do cause occasional problems during high RPM operation. It is important to use properly clearanced rotor bearings. In general, do not replace rotor bearings when you are re-using rotors unless they are over the recommended clearance or show some damage from dirt, heat, or lack of oil. Our experience has shown that used bearings in good condition are safer than new, untried bearings.

For high performance street and racing use, the best rotor bearings we offer are Mazda Factory Race (MFR) rotor bearings with a deep oil groove and additional clearance (approximately .0005" extra). While stock bearings can be grooved deeper and polished to obtain additional clearance, the MFR bearings are still the better choice. The MFR groove is approximately .070" deep to provide an improved reservoir of oil around the bearing.

Up to 8,500 RPM, .003" +/- .0005" eccentric shaft-to-rotor bearing clearance is acceptable. Beyond 8,500 RPM, clearance should be .004"+/- .00025". Clearance can be adjusted by polishing the inside of the rotor bearings with 400 grit or 600 grit wet-or-dry sandpaper strips which are spun with a high speed motor such as an air powered die grinder.