WARNING: This page is about a different car, the 2011 BMW 750xi, 2011 BMW 750Lxi, 2010 BMW 750xi, and 2010 BMW 750Lxi. However, it is still accessible from the selected car via links, so may be relevant.
0491388 170004 Adapter MW
NOTE:
(Adapter) For screw-on radiator caps (standard thread)/ Series: E28, E30, E32 / Engine: M51 Free only for motorcycle!!! [Discontinued]