Lewmar's plain-bearing Synchro Blocks are engineered for speed, efficiency, and superior handling. Lewmar uses a combination of scientifically optimized block geometry, a free-spin bearing, and a swivel lock self-aligning head to reduce friction and increase efficiency by up to 40% over budget blocks on mainsheet systems. Each component is perfectly synchronized with the movement of the rope, providing you with an easier, smoother transfer of power from deck to sail and less wear on your rope. The toggle heads of the halyard blocks are designed to fit on the studs commonly found at mast bases - check the diameter of the stud against the width of the block jaws and pin diameter.