Is there a consensus as to when flex discs should be replaced? typical mileage, etc? I don't believe, based on maintenance records that my wagon has had these replaced or even "looked" at...just something to add to the list I guess.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)