A brief lesson from the world of physics: heat expands and contracts the parts in your cooling system, and the expansion tank in your Z4 gives the coolant somewhere to go during heat cycles. Made from plastic, these tanks are exposed to incredible heat and pressures, causing the hose fittings and joins to fail without warning. With the high failure rate of factory tanks, we recognise that the market has been flooded with notoriously cheap aftermarket replacements. We choose to stock BEHR tanks that are OEM quality and designed to last.