WARNING: This page is about a different car, the 2010 Audi A8, 2009 Audi A8, 2008 Audi A8, and 2007 Audi A8. However, it is still accessible from the selected car via links, so may be relevant.
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Tighten bolts in 3 stages as follows:
Tight bolts by hand in a diagonal sequence.
Pre-tighten all bolts in a diagonal sequence to 5 Nm.