One post said that if they are cracked they need to be replaced...this is true. However one said if it is not cracked they are ok My experience says otherwise. My 1993 190E needed a front flex disk at 104k and it looked ok "visually". Rubber even looked pretty fresh. Once it was removed though, the metal sleeves that the bolts go through were jarred loose and allowed significant play! My symptoms were the feeling of slack if I accelerated and let off suddenly, and there was a good clunk on easier, low rpm shifts.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000