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Clutched alternator pulley upgrade for the M57 engine used in the 335d.
A clutched alternator pulley dramatically reduces the stress on pretty much all engine components. Without a clutched pulley engine pulsations conflict with the static-load of the alternator, causing snapping of the serpentine belt. This can drastically reduce the life of accessory/serpentine drive components like the tensioner, A/C compressor, power steering pump, harmonic balancer and alternator itself.
Not putting a clutched pulley on the M57 engine was a mistake that BMW made purely for cost savings and causes premature failure of engine components, especially the harmonic balancer. See our article on How Clutched Alternator Pulleys Work.
Please note that this pulley/spacer combination is only for the 335d. We carry a different pulley/spacer for the X5d.