BMW changed the alternator belt tensioner design from a failure-prone, less reliable mechanical setup to a hydraulic unit for all their latest models.
In fact, BMW service will only replace your tensioner with the updated design, as the older unit has been discontinued.
If you are experiencing problems with your tensioner, or are replacing your alternator belt and want the latest, most reliable setup, ECS offers Hydraulic Alternator Belt Tensioner Conversion Kits to get the job done.
Each kit includes everything required for a thorough conversion to the new hydraulic system.
Click here for more info on E83 X3 3.0L Hydraulic Alternator Belt Tensioner Kits