Abstract: A rock bit for drilling subterranean formations has improved dynamic O-ring seals for retaining lubricant around the rock bit bearings during operation at more than 50 meters per minute surface speed on the O-rings. Such a bit has a plurality of journal pins, each having a bearing surface, and a cutter cone mounted on each journal pin and including a bearing surface. A pressure-compensated grease reservoir is in communication with such bearing surfaces for maintaining a grease adjacent to the bearing surfaces. The grease is sealed in with dynamic O-ring seals comprising a perfluorinated terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and a cure site monomer for introducing crosslinking sites. Preferably the cure site monomer is selected from the group consisting of trifluoroethylene, vinyl perfluoroalkyls and monomers including a perfluoroalkyl ether group.