These are perfectly sized for use with 1/4" line. Catalina Yachts uses them for 5/16" line also. When used with 5/16" the line does touch the smooth frame on either side a bit. Not enough to cause a problem, but still it does touch. The advantage is, the blocks only require a narrow slot in the mast, much smaller than a Harken double exit block for example.

Approximate Dimensions:
Mounting hole size:
• Height: 2-1/8"
• Width: 1-1/16"

Fastener hole spacing center to center:
• Top to bottom: 1-3/4"
• Side to side: 1-1/2"

Fastener hole size:
• 3/16"

Overall size at mounting flange:
• Top to bottom: 2-1/8"
• Side to side: 1-7/8"

Sheave dimensions:
• Diameter: 1-1/2"
• Width: .40
• For line diameter: 1/4" (was often used by Catalina Yachts with 5/16" line)