40mm Dynamic Single Swivel Block.New for 2023.Made from high-strength, lightweight glass-loaded nylon and featuring AllenÕs dynamic bearing technology for a high-performance block suitable for keelboats and dinghies.40mm dynamic blocks are perfect for use on main, jib and spinnaker sheets on dinghies and keelboat control lines.Bearings Ð Dynamic blocks feature our patented bearing system which uses twin rows of stainless-steel ball bearings running on a stainless-steel rail. The ball bearings are caged for further friction reduction. These features provide a superior load-carrying ability and longevity when compared to other bearing systems.Swivel Head Ð Featuring a 360-degree swivel head with a 4mm shackle. The shackle is load rated higher than the block, so although it might look small, it's plenty strong enough!Becket - The 40mm range features a large central hole, which can be used as a becket (tying-off point). Specific becket variations are also available.