Of all the components that make up this Valve(s) the O Ring,16mm has the lowest price.
The fifteen Ex500d8 Ninja 500r 2001 Usa California Canada Valve(s) component parts are all listed in this schematic.
A sample from the products for the Valve(s) include: Retainer-valve Spring, Seat-spring,12.1x16.2, Seat-spring,17.5x23.8, Collet.