My 300 and 420 has 200,000 miles a piece with original timing chain on each. Both run like a charm, no noise. (Except the 300 needs power steering fluid).

What is the risk of changing the timing chains when they simply give rather than changing them now when everything now is fine?

I know with some 4 cylinder cars you can bend the valves because some pistons can hits the valves (my understanding) when the timing belt breaks, but if you have clearance on the 6 cyl and 8 cyl I don't see the benefit of pre changing your timing chain before any symptoms?

What are some symptoms of when you chain is giving? I heard that the Tach will flucuate at idle. So far the needles are stready.