This 5 Inch aluminum and rubber backer is 5/8-11 Thread. Rubber Backer’s max capacity / velocity is 5000 RPM. Please do not use it with high speed grinder or polisher. This package carries (a)-(b). (a) 15 Piece of 5″ / 125mm (5/8″-11 Thread arbor) Aluminum Backer / Pad holder for diamond polishing pad. (b) 11 Piece of 5″ / 125mm (5/8″-11 Thread arbor) Rubber Backer / Pad holder for diamond polishing pad. Rubber and aluminum Backer’s max capacity is 5000 RPM, please don’t use it with high speed grinder / polisher.