For the person asking why the need to change the expansion tank:

Bmw engines run very hot and under lots of pressure. The expansion tank is plastic and pressurized, as you know. Those things combined cause the plastic to weaken over time and eventually they crack while in use or even explode. If you have no idea when the last time yours was changed, I would change it so you have baseline.

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2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles