57mm Aluminum Low Load Black Magic Block with SwivelBlack Magic Air blocks are a top favorite of Big Boat performance racers and cruisers. These lightweight, free-running, powerful performers are used for sail controls that see lots of action: mainsheet, runner, halyard, and spinnaker systems. A center cage keeps TorlonA(R) roller bearings separated and parallel to reduce friction. Shackle blocks lock in two directions or swivel to keep the line from twisting. Low-load blocks have red isolators. They use TorlonA(R) rollers and 316 stainless steel shackles. Standard blocks have 17-4 PH stainless head posts. Torlon is a registered trademark of Solvay Advanced Polymers L.L.C. Specifications: Sheave: 57mm/2-1/4in Length: 119mm/4-11/16in Weight: 154g/5.44oz Shackle Pin: 6mm/1/4in Maximum Line: 12mm / 7/16in Maximum Working Load: 7/16kg / 1655lbs Breaking Load: 1500kg / 3300lb