What's the Difference between a Flywheel and a Flexplate in Diesel Engines?

flywheel diagram clutch disc pressure plate | Highway & Heavy Parts


We get a lot of people wondering what the difference is between flywheels and flexplates.

While flywheels and flexplates have similarities, they are not the same. People often refer to them as the same part, but while flywheels and flexplates help accomplish the same task, they do it in different ways.

This depends on whether its an automatic or manual transmission.


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What's a Flywheel For? 

Flywheels are used to mount clutches on manual transmission vehicles.

A manual transmission has a flywheel that is attached to the crankshaft and has a clutch disk in between the pressure plate and flywheel.  The flywheel is connected directly to the clutch, allowing torque to transfer between the transmission and the engine. The flywheel can provide a friction surface for the clutch to contact.

Needing to replace your crankshaft? Check out our crankshaft installation guide!

Myodesie.com shares the three functions of a flyweel: "First, through its inertia, it reduces vibration by smoothing out the power stroke as each cylinder fires. Second, it is the mounting surface used to bolt the engine up to its load. Third, on some diesels, the flywheel has gear teeth around its perimeter that allow the starting motors to engage and crank the diesel."

A flywheel is a heavy wheel that takes a lot of force to spin around. When it's spinning at high speed, it can store a great deal of kinetic energy. Its job is to store rotational energy that produces power to the transmission. It has a good bit of mass to store energy to keep the engine turning between pulses at idle.

Flywheels vary in size. It could be a large-diameter wheel with spokes and a very heavy metal rim, or it could be a smaller-diameter cylinder made of a carbon-fiber composite sometimes with steel rims.

Their life cycle can outlast that of the clutch, but if you're installing a new clutch, it will need to be resurfaced.


Signs of a Bad Flywheel

There are some signs and symptoms to look for that will let you know when the flywheel needs to be replaced. Here are 3 common signs:

  • A burning smell: This smell comes from too much heat produced by the clutch facings. "Riding" the clutch while driving causes this burnt toast odor. This causes the flywheel to wear out prematurely. 
  • Gear slippage: You'll notice this while you're driving. If the transmission can't shift to the next gear, it will slip into the previous gear. When caused by a bad flywheel, gear slippage causes plate grinding, and the driver or a mechanic may find small metal shavings in the transmission fluid.
  • Clutch vibrations: You'll feel this in the passenger compartment of the car while driving. They are typically caused by the spring mount mechanism on the flywheel failing, and will greatly reduce the flywheel's performance when the driver depresses the clutch.


What do We Need a Flexplate For?


flex plate diagram engine components | Highway & Heavy Parts


Flexplates are used to mount torque converters on automatic transmission vehicles. 

Torque converters replace the clutch of a manual transmission. A flexplate is mounted to the crankshaft and connects the output from the engine to the input of a torque converter. The torque converter is located between the flexplate and the transmission.

A flexplate is typically a stamped steel disc with a ring gear welded on that mounts the torque converter to the rear of the engines crankshaft. Flexplates are generally much thinner and lighter than a flywheel because of the elimination of the clutch surface.

The metal frame of the flexplate has multiple uniform machine-cut holes for mounting to the crankshaft. The other holes are specific to the vehicle, torque converter set-up, and potential weight balance of the flexplate.

While there are some exceptions, if your vehicle has a manual transmission, you want a flywheel, and if your vehicle has an automatic transmission, you want a flexplate.

Need help with your diesel engine? Our ASE Certified Techs have answers for you! Call them at 844-304-7688. You can also request a quote online!


Originally posted October 8, 2015; Edited September 24, 2019