WARNING: This page is about a different car, the 2005 BMW 645Ci and 2004 BMW 645Ci. However, it is still accessible from the selected car via links, so may be relevant.
Fit conrod with piston pin to piston in such a way that arrow on piston points to front and flat end of conrod points to rear.
Fig 1: Fitting Conrod With Piston Pin To Piston (Cylinder Bank 1 To 4)