Top radiator hose defect on E34 M5:
I had a new OEM BMW top radiator hose that leaked from the reinforcing cords where they were exposed on the ends of the hose. This occurred on 3 replacements from the dealer as well. A closer look showed that the molded tee fitting was defective – there was exposed cord on the inside of the hose, allowing coolant to follow the cords out the end of the hose, even past tight hose clamps. BMW NA purchasing looked at this issue, but said they did not have a high enough reject rate on the parts to indicate a problem!
Picture of the outside molding for the small tee branch:
Picture of the inside, (arrow) showing the exposed reinforcing cords: