Harken Zircon blocks feature industrial ceramic ball bearings contained in caged races to ensure maximum efficiency. With sizes ranging from 40-57 mm sheaves and the option for single or double blocks, these blocks are the most efficient choice for smaller boats. Becket Part No. 2191 has a maximum working load of 115 kg (250 lbs) and can accommodate line diameters up to 8 mm (5/16 in).