Was just reading another thread about a guy finding a 91 300D in a barn, and a poster chimed in that the timing belt on some 602, 603, and 606's ran the possibility of excessive timing chain stretch. One symptom was rough running. I'm waiting for my engine mounts to get here so I can replace them, thinking that's why I have the rough idling. BUT, If that doesn't cure it, would the chain be the next place to look? I have 215k on the car. I hope the mounts cure the roughness. Do the chains go "bad"?

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide