Lewmar's Synchro blocks are more efficient, less expensive Lewmar's Synchro blocks combine geometry, sheave ratios and a unique free spin bearing, to deliver performance that s comparable to ball and roller bearing blocks used in pro racing at a more affordable price. With conventional blocks, one of the biggest losses in efficiency comes from the distortion of line as it wraps around the sheave. Synchro blocks provide the optimal ratio between line size, sheave diameter and the interface between rope and sheave. The result is larger sheaves with a smaller axle than on conventional blocks. For best performance, use the line sizes marked on sheaves. Synchro blocks improve efficiency by nearly 5% on a single block at a load of 442lb (200kg). Calculated over a typical six-block mainsheet system, this approaches a 40% improvement over conventional blocks. This new range of blocks transforms performance, not only by improving efficiency, but also speed and handling. Optimized block geometry, a revolutionary free spin bearing and a self-aligning head combine to reduce friction. Each component is synchronized with the movement of the line providing an easier, smoother transfer of power from deck to sail, and all with less line wear. Sheave: Reinforced plastic Ball Bearings: Free spin Sheave Size: 105mm (4-13/16") Line Dia.: 5/8" Safe Working Load: 7,700lb. Weight: 27.51oz. Cheek Material/Finish: Stainless steel and plastic Shackle/Strap Type: 15 swivel