Basically a flywheel or flex plate is the round metal disc shaped component that transfers power from the engine to the transmission. A flywheel is usually referenced to a car with a standard transmission,and a “flex plate” is used with automatic transmissions.
The flex plate bolts to the engine crank shaft and to the torque converter of your automatic transmission. When the engine starts and is running the flex plate turns, which turns the torque converter and powers your hydraulic pump in your transmission. They sometimes will crack from just metal fatigue and vibration.
On the outer circumference of the flex plate or flywheel is a “ring gear” The ring gear is what your starter motor uses to crank the engine to start it. A manual transmission uses a “flywheel” A flywheel is much thicker and heavier than a flex plate and stores energy to keep things smooth while shifting gears. It also supply’s a friction surface for the clutch disc. A “flywheel” usually does not crack and fail like a flex plate.
A flywheel can crack on the friction surface from heat caused by a slipping clutch. The ring gear on a flex plate and a flywheel can fail. They can wear from normal wear and tear, but usually fail because the teeth on the gear are ground away or even break off from a bad or week starter motor. The “ring gear on a flywheel can usually be replaced. The ring gear on a flex plate requires replacing the “flex plate assembly”.
Either way your transmission will have to be removed to do the repair which is very labor intensive. If you hear a grinding noise while starting your vehicle, chances are you have a starter motor that is failing. Replacing the starter motor is important to protect the teeth on the flywheel and possibly prevent it from cracking. A cracked flex plate can also turn on your check engine light, cause stalling and poor running symptoms.