The O-ring,7.8x2.2 is the lowest priced item on this Gasket Kit A schematic.
On this parts fiche the Seal,valve Stem is the most specified component.
A sample from the products for the Gasket Kit A include: Gasket,ex Pipe (nas), Gasket,breather C (nas), O-ring,7.8x2.2, Gasket,cylinder H (nas).