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Summary
  • The flex plate allows for this expansion of the torque converter. It helps start the engine by providing rotational power and connecting the output of the engine to the torque converter. 
  • Flywheels, on the other hand, are plates used in vehicles with manual transmissions. They help the starter motor power the engine and provide the clutch with a surface to latch onto.
  • Flywheels also help balance the crankshaft. The very heavy torque converter with its internal fluid dynamics more or less does balancing dynamically in addition to being balanced itself.
  • The two are distinct in their use for different vehicle types. Flex plates are also thinner and lighter than flywheels.

Many drivers believe that flex plates and flywheels are similar enough to be interchangeable. Though they are arguably among the most important parts of any vehicle transmission, flex plates and flywheels are quite different, but they do typically have a few things in common.

Flywheel vs Flex Plate

They might perform similar roles and are connected to similar components, but the flywheel and flex plate aren’t interchangeable. Let’s take a closer look at how these two components function to better understand how they differ.

What is a Flex Plate?

image of a car flex plate
A flex plate | Image Source: Richard McCuistian

When the engine accelerates, the torque converter is filled with fluid pressure and expands just a bit by design. The flex plate allows for this expansion of the torque converter. This is why flex plates sometimes crack around the bolt circle where they bolt to the crankshaft. The repeated flexing causes them to break like any other piece of metal does when it is repeatedly flexed.

The flex plate is bolted to the rear of the crankshaft with a circle of bolts at its center, and is bolted with from 3 to 8 bolts to the torque converter around its outside edge. Most flex plates have the ring gear for the starter drive around their outside edge, but sometimes this gear is part of the torque converter and the flex plate is nothing more than a flex plate.

What is a Flywheel?

automotive flywheel image

Much like flex plates, flywheels are bolted to the crankshaft in the center and the clutch pressure plate is bolted to its outer edge. The flywheel is thick and heavy and has a machined surface for the clutch disc.

“Dual Mass” flywheels are built from two or three different plates with springs for balance and vibration dampening.  The ring gear for the starter is just about always pressed onto the flywheel.

Unlike a flex plate with a ring gear welded to it, the starter ring gear can be replaced on a flywheel by knocking the damaged ring gear off and heating the new ring gear slowly while it settles into place, then letting it cool, much like blacksmiths used to do when installing steel tires on wooden wagon wheels.

What is the Difference Between a Flex Plate and a Flywheel?

The biggest difference between flex plates and flywheels is the fact that the two parts, though similar in many regards, were designed to work with two completely different transmission types.

Flex plates are used with automatic transmissions whereas flywheels are used with manual transmissions.

There are other key differences between the two. For example, flex plates are thinner and lighter than flywheels.

Signs Your Flywheel or Flex Plate is Bad

It’s not a good idea to drive a vehicle with a bad flex plate or flywheel because these components help you control your vehicle’s speed and power output.

The caveat is that flex plates are more failure prone than flywheels, but flywheels can be damaged to the point of needing replacement if the clutch wears to the point that it ruins the surface of the flywheel. Machine shops, however, can resurface flywheels. You just need to do a cost comparison.

Flex plates are more failure prone than flywheels, but flywheels can be damaged to the point of needing replacement if the clutch wears to the point that it ruins the surface of the flywheel. Machine shops, however, can resurface flywheels. You just need to do a cost comparison.

Richard McCuistian, ASE Certified Master Automobile Technician

Because flex plates and flywheels are such crucial components, it’s a good idea to keep an eye out for the signs that it’s time to replace them.

, Flex Plate vs Flywheel: What’s the Difference?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Of course, you can’t get a good look at the flywheel or flex plate without removing the transmission, but you can hear a cracked flex plate and it may change sounds when the transmission is in gear.

Clutch Pedal Vibrates

Flywheels in manual transmissions absorb vibrations when driving, keeping the clutch pedal steady. If it gets damaged, its ability to keep the clutch pedal stable may become compromised.

If your clutch pedal suddenly starts vibrating when you drive, you may need to check if your flywheel is cracked, bent, or damaged in any way. Note, however, that this symptom more likely points to a problem with the clutch disk.

Hard Starting

A bad flywheel or flex plate may affect the operation of your vehicle’s starter. It’ll be more difficult to start the engine and you might hear whining noises during startup.

It’s rare, but sometimes the flywheel ring gear will get loose so that the starter spins only the gear instead of the flywheel and the engine.

Strange Noises

Cracked plates are loud and make repetitive knocking sounds often mistaken as rod- or main-bearing noises. If you hear any unusual noises coming from the crankshaft, it’s worth investigating in case there’s anything wrong with the flywheel or the flex plate.

damaged flex plate
The flex plate pictured cracked around the bolt circle where it attaches to the crankshaft. | Image Source: Richard McCuistian

Vibrating Engine

If the flex plate or the flywheel is unbalanced or improperly bolted onto the vehicle, your engine may rattle and vibrate. Dual mass flywheels with multiple plates and springs are prone to this when they wear out. This can be dangerous because the more the engine rattles, the more likely it is for either the plate to come loose.

Both flywheels and flex plates are heavy and pack a lot of energy, meaning one that comes loose can seriously damage your vehicle.

Slipping Clutch

Drivers of vehicles with manual transmissions might experience clutch slipping when the flywheel gets warped or damaged in any way.

Slippage occurs when the flywheel fails to engage with the clutch properly, typically because there’s an oil leak that loosens its grip or if it gets bent out of shape.

Again, however, note that this is usually a clutch problem rather than a flywheel problem, and if the flywheel is damaged, the clutch is usually what does it. 

What Causes Flex Plates and Flywheels to Break?

Expansion of Torque Converter

Flex plates break because they flex when the torque converter expands. This typically has nothing to do with the way the vehicle is driven other than pulling a lot of heavy loads.

Bad Driving Habits and Collision Damage

Driving with an overloaded vehicle is a bad habit that can contribute to damaging the flex plate or the flywheel.

For vehicles with manual transmissions, riding the clutch and driving in the wrong gear may also cause serious damage to the flywheel.

Lastly, it is also fairly common for impact damage from a collision to damage transmission components.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Engine , DIY Tagged With : ,
Garage Essentials
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