MReid:
A 420SEL timing chain is old and dangerous at 100000 miles, we typically recommend replacement at or before then. The chains themselves rarely break but slap around and cause the failure of one of the nylon(plastic) guides in the head and immediately snowballing into big money, read $3500.00 plus. Use a Mercedes only tensioner and change it also along with tensioner rail and upper guides. Typical cost $750.00 or so