This is a single Phil Wood 6001 Cartridge Bearing. Compatible with 2nd Generation Front Track & Road Hubs, Surly Track Hubs. Features: Phil Wood and Company is credited for pioneering the first production sealed bearing cycling components 100% filled with Phil Waterproof Grease for maximum durability Bearings with an "X" have the same seals as those specified for submersible motor/pump bearings Specs: Bearing: 6001 ID X OD X W Mm: 12x28x8