Earlier models (R24 - R26, R51/2 - R67/3, R50 - R60) used flat-faced cam followers in combination with camshafts featuring concentrically aligned lobes. Later models (R27, R50/2, and R60/2) introduced convex cam followers to promote rotation, used in conjunction with camshafts having eccentrically positioned lobes. New flat-faced cam followers are no longer available; all current replacements are manufactured with a convex contact surface. Convex cam followers can be used with both concentric and eccentric camshafts. However, flat-faced cam followers must not be used with eccentric camshafts.