The biggest reason for the flex plate being cracked or broken is the adapter that is bolted to the back of the engine being out of alignment.

You can use a dial indicator on the crank to see if the flex plate is out of true using the crank as the reference.

It sounds to me as if someone forgot to tighten the torque converter bolts.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS