300-34 Clamp on Furling lead block. Delrin(tm) ball bearing sheave allows for quick and easy furling line adjustment. Stainless steel cage and strap ensure durability. First stanchions 1" (25mm) in diameter. 1-7/16" (37mm) Sheave Diameter, Maximum 7/16" (10mm) line size. Safe Working Load = 1000 lbs (450kg)