I recently had to replace an idler pulley on my Z4 M, and learned a couple things along the way.. Most of the information is here:
http://www.bimmerfest.com/forums/sho...d.php?t=462797
One important thing to note is based on my experience, and extensive research, is that the S54 engine tensioner pulley on the alternator/water pump seemed to jump back and forth between a T50 torx and 8mm socket head screw. This is what you need to loosen the tensioner off the belt. Really annoying if everything you read is 8mm socket head and you need a T50 torx, so I'd recommend having both.
ECS tuning has most of the belt drive components.