Connecting the lower radiator to the thermostat. Over time, rubber hoses can become hard, with the internal walls covered in sediment and the plastic fittings turning brittle or the internal o-rings simply leaking. A must replace hose for any E60 5-Series owner looking to preventively maintain or restore their cooling system. We've found these Rein hoses to be the best quality, which is manufactured by the original equipment supplier and use BMW Genuine plastic hose connectors.