BK45-1" Sheave Cast iron Pulley - OD: 4.25" for V-belt size B Class 5L (BK451)BK45 1" Sheave cast iron Solid Pulley With OD: 4. 25" inch for V belt size B Class 5L (BK451) BK45 1" BK Finished Bore Sheaves for (4L or A or 5L or B Belts) BK Sheaves are accurately machined from fine grained cast iron, statically balanced, They are furbished with Standard Keyseats and Hollow Head Setscrews The V Belts you that can run in this pulley are 4L, or A or 5L, B, V Belts. The belt series most common for this pulley is the 4L and A section.