In the Catalina 350, Catalina 380, Catalina 387, Catalina 400, Catalina 42, and Catalina 440 mastheads there is a pin along with Delrin bushings that support the sheaves. This replacement sheave doesn't require the Delrin bushings.

Below is a Inventory of the usage:
• (4) per Catalina 350, 380, 387, 400, 42, 440 masthead in-mast furling rigs
• (4) per Catalina 350, 400, 42 masthead non-furling rigs

Replacement injection molded sheave dimensions (#F8128) has a smaller ID and doesn’t require the original Delrin bushings, but rather is supported by the original 1/2" pin only:
• Outside diameter: 85 mm (3 3/8")
• Inside diameter: 12.5 mm (~1/2")
• Width: 15 mm (~5/8")
• Line diameter: 13 mm (~1/2")

Original Acetyl Sheave Dimensions (#T1961):
• Outside diameter: 85 mm (3 3/8")
• Inside diameter: 22 mm (~7/8")
• Width: 15 mm (~5/8")
• Line diameter: 13 mm (~1/2")