The O-ring,2.5x1.5 is the lowest priced item on this L. Front Brake Caliper(cb 1000ra) schematic.
Some items from this specific schematic: Piston,25 B, Boot B, Seal Set,piston, Pad Set,fr (nas).
This schematic depicts all components used in the Cb1000ra 2011 (b) England / Abs Mph L. Front Brake Caliper(cb 1000ra) partslist.