Harken 25mm Fly Block (1250.ILCA)The 25mm Harken Fly Block is designed to provide strength without mass, providing sailors with big power in a low aero package. Designed specifically for high tech lines, these efficient blocks have an incredible working load for their small size. The high strength cast stainless steel boom key is uniquely shaped and angled to minimize weight, and remain in place, even under zero tension. Specifications Sheave (mm) 25 mm Sheave (in) 1 in Length (mm)