The Service Bulletin I mentioned cited complaints of hard starting, rough running, and smoke/no power. Larry Bibles' description of checking chain stretch is right on the money, however MB says that reading the degree marks on the balancer is conclusive enough to decide whether or not to replace the chain. I'm not sure what the diffrence is, but I've replaced dozens of 603 chains, but I've yet to see a bad 602/601 chain. Probably the same chain set-up, but the 4 and 5 cylinders are less taxing on the chain. Aside from the the chain stretch issue, I've yet to see a bottom end failure of any kind on the 603, even at 400K+. It's a well built motor capable of exceptionally long life. With that said, my favorite is still the 617.