Full Ceramic Silicon Nitride: Si3N4 Grade 3 balls and races . Silicon Nitride is the hardest, most resilient material a bearing can be made from. Specially designed seal on the element side creates the lowest friction possible, internal side has a the PTFE retainer. Packed with Kyodo Yushi PS-2 grease. Requires proper installation tool, AB-9055. Available installed through the BTI Factory Service Center