Courtesy of G37sHKS and his current built motor & transmission thread, he posted a good article on Flexplate crack:
AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - What Causes A Flexplate To Crack
Quote:
Slight misalignment may only cause a single flexplate to break in the life of the vehicle. More severe misalignment can break a flexplate in 15,000 to 30,000 miles. Excessive crankshaft flange run-out or end plate can also cause flexplate cracking. This may occur in worn engines or engine that have been poorly rebuilt. An engine, transmission or torque converter that has been dropped may also have damage that will cause misalignment.
When a running engine ingest water or coolant, the crankshaft may be bent. Liquids will not compress and immense force is placed on the crankshaft and rods. This sometimes happens with a leaking intake manifold or driving on a flooded street. The bent crankshaft will have run-out at the flange and may have continued problems with flexplate cracking.
The same things that cause the flexplate to crack can also cause transmission problems. Damaged and leaking front seals and front transmission pumps are common symptoms.
When a flexplate cracks, it often produces a loud knocking noise as it rotates. The sound is very similar to a bad engine rod and is often misdiagnosed. Removing the bolts to the torque converter and sliding it back into the transmission will often reveal the real problem.
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