20mm single block as used on the RS21. Technical features of the BBB20 Tie On and Cheek block: -50% Glass fibre filled composite cheeks. Acetal ball bearings for lightweight. Glass fibre reinforced sheave. Sheave diameter: 20mm. Breaking Load 661lbs, 440lbs for Cheek and Tie On. Max Line: 6mm. Bolt Size: 4mm.