Yeah ones that have a groove worn into them are better. More surface area for the balls to grip on so they won't slip, and they keep getting smoother.
I've got an RC10T2 (same diff as the B4) which I got new in 1995, it's done more running than any other RC car i've got, running on the original diff rings with nice grooves worn in them, and the original carbide balls, and it's the smoothest diff i've got.
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