Hey everyone! I am the proud new owner (although not as much as of yesterday), of a '78 240D 128,000MI. While driving it home at about 65MPH, heard a noise and subsequently pulled over at a truck stop. Turns out, crank pulley sheard where the 2 v grooves meet, tore up rad and rad fan blades. My question is, what in God's name could have caused this?? Fatigue due to rust as there was SOME rust built up on the inside or some other reason? Please help as this really put a damper on the purchase of this beautiful machine. What steps do I follow for the re&re?

Thank you all to those who can help!