[Wgosma]

Thanks for the details. I have 2012 X3 (N52 motor) w/58K miles on the clock and a new serpentine belt sitting on the workbench, I was planning to replace ONLY the belt.

Upon examining my car the belt sits 'off' the tensioner nearly identical to the pic you've posted. So seems it's time for new tensioner w/idler pulley in addition to the new belt.

Question: if the reason the belt is not properly positioned on the tensioner's idler pulley is due to 'sag' (stretch) of the aluminum bolt which secures the tensioner to the head, why could one not simply remove the drive belt and replace the aluminum bolt? Or....is there something internal to this mechanical tensioner that also contributes to the problem? ...or perhaps more likely the bearing in the pulley is simply worn?
Thanks/Bill