Belt Tensioner - E9X M3, E6X M5/M6 - 11287835417
Replace the tensioner with every other belt change.
The belt tensioner maintains the correct tightness on the serpentine belt even as it ages and becomes longer. This one is made specifically for the S65 V8 and S85 V10 engines, as found on 2008, 2009, 2010, 2011, 2012 and 2013 model BMW E90/E92/E93 chassis M3, as well as 2006, 2007, 2008, 2009 and 2010 model E60 M5 and E63/E64 M6.
This tensioner self-adjusts so that the accessories on the belt (water pump, power steering pump, and alternator) are always working within spec. The M3/M5/M6 belt tensioner will need to be replaced if it can no longer adjust the belt on its own. If you're planning any belt service on either an S65 or S85 M engine, consider replacing the tensioner for peace of mind. A new tensioner should be installed on every other belt change (or every 80-100,000 miles).
E90 / E91 / E92 / E93 (06-13) > M3
E60 / E61 (04-10) > M5
E63 / E64 (04-10) > M6