Harken 57mm Aluminum Low Load Black Magic Block w/Swivels. 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 Torlon 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 Torlon rollers and 316 stainless steel shackles. Standard blocks have 17-4 PH stainless head posts. Package Contents: Harken 57mm Aluminum Low Load Black Magic Block w/Swivel Harken 3226: 57mm Aluminum Low Load Black Magic Block w/Swivels