A brief lesson in physics: heat expands and contracts the parts in your cooling system. What the expansion tank does is give the coolant somewhere to go during heat cycles. Made from plastic, over time these expansion tanks can become brittle and crack. The only solution is replacement. Made by original equipment supplier to BMW, Mahle (BEHR), this tank comes complete with a level indicator and bleed screw. 

OEM Part Number: 17137501959
NRF Part Number: 454036