1/8 Ceramic Diff Balls
This is Avid Rc 1/8" Silicon Nitride (Si3N4) ceramic balls are 30% lighter than steel and 40% stronger. Each ball is over 1000X more round than the best steel bearing of any type, grade or benchmark. In other words your diff will be smoother and last longer due to the balls not deforming and causing havok to your rings.
These 1/8" (3.175mm) ceramic diff balls are commonly used in the spur gear of 1/10th and 1/12th scale cars. Please check your manual for the correct size.