A flywheel bolts to the crankshaft and is typically used with a clutch with a manual transmission. It serves as the friction surface against one side of the clutch disc. It has a good bit of mass to store energy to keep the engine turning between pulses at idle.

A flexplate is very thin and also bolts to the crankshaft. It serves to mount the very heavy auto trans torque converter. The weight of the torque converter provides the mass as described above.

Both of them have a ring gear for engagement of the starter.

Hope this helps
Larry

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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