Every vehicle on the road has a coupler between the engine and transmission, for an automatic transmission this is called the flexplate, for a manual transmission it is called a flywheel.

A flexplate is what connects the engine to the automatic transmission in a vehicle. It is basically a piece of thick sheet metal that bolts to the end of the crankshaft and to the torque converter. They are made of a thinner material than a flywheel to allow for expansion of the torque converter at higher ring gears engine rpms. Some flexplates have a toothed ring on the outer edge for the starter to engage with, others have the toothed ring on the torque converter.

Flexplates are engine specific items due to the fact that some engines require external balancing to smooth out vibrations, these flexplates will have a weight welded to it somewhere, others will not. If these are mixed up, the engine will have a serious vibration that could actually cause internal engine damage if left unchanged.

If your vehicle has a manual transmission, then it uses a flywheel instead of a flexplate. Flywheels are much heavier than a flexplates. Flywheels come in different weights depending on the application. A lighter flywheel would be used in high-performance applications where higher rpms are the norm, a heavier flywheel is used in trucks and other low rpm applications where you need the extra help just off idle.

The flywheel is where the clutch assembly mounts and is the contact-friction surface for the clutch disk. Whenever a clutch is replaced in a manual transmission vehicle the mating surface on the flywheel needs to be resurfaced and measured to be sure the thickness is within specification, otherwise the flywheel can crack and even break in extreme cases.

Flywheels are where the toothed ring is mounted for the starter motor to engage. In certain applications this ring can be replaced instead of replacing the entire flywheel when the teeth become damaged. If you need to replace your flywheel, always use a high quality replacement part such as the OEM (Original Equipment Manufacturer) parts. They will fit right and perform better than less expensive aftermarket products.