WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. However, it is still accessible from the selected car via links, so may be relevant.
0491387 170003 Adapter MW
NOTE:
(Adapter) For screw-on radiator caps (standard thread) / Series: E28, E30, E31, E32 engine: all, except M51 Free only for motorcycle!!! [Discontinued]