On a U.S. spec 1980 M117 450: I replaced the timing chain on this car about 120,000 miles ago, and now it’s time to do it again. Can 10 (crankshaft) degrees of retard on the passenger side camshaft be attributable to chain stretch only, or does it signify further problems such as worn out sprockets? Is it critical to replace the tensioner rail as well as the uppers? I am planning on running the new chain through, and need to go shopping…THANKS TO ALL!