Series 40 All-Purpose Fiddle Block. Ronstan's 40mm plain bearing blocks are strong and compact. Smart features include a unique captive lock universal head. For dinghies and keelboats up to 26'. 1 9/16" (40mm) sheave is made from UV-stabilized acetal and accepts a line size of 5/16" (8mm). Cheek is made from stainless steel and fiber-reinforced textured Zytel.