Series 40 Ball Bearing Blocks are suitable for sheet applications on dinghies, sportboats and small one-design keelboats, and for control lines on larger boats. They feature a precision two-stage bearing system for better durability, lighter weight and low-friction performance. Models with swivels feature stainless steel shackles.Sheave: Carbon fiber reinforced, PTFE impregnated engineered nylonBall bearings: High-compression-strength carbon black acetal; Stage 2 bearing: Glass-fiber-reinforced MoS2 impregnated engineered nylonCheek material/finish: Toughened glass-fiber-reinforced nylon