Ronstan's rope sheaves leverage the self-lubricating properties of Acetal to deliver high strength, durability and excellent low friction properties for bearing applications. Moulded from a specifically modified ultra-violet resistant grade of material to ensure a long service life in the harsh conditions of a marine environment. UV resistant.3/4" x 3/8" (19mm x 9.5mm)Inside opening: 5/16" (8.2mm)