Stock O.E.M. replacement part Rancilio Silvia Steam Wand/Valve Shaft O-ring. In the pre-2008 (Versions 1 and 2) Silvia, you will more than likely need two each of these for both the steam wand and/or valve. In the post-2008 (Version 3) only two are needed for the steam valve, and they are not used in the wand.
As with any o-ring and moving parts, lubrication is highly recommended