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What is a flywheel and what does it do?

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Updated 11 Oct 2019

Jason Unrau

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A manual transmission might seem like a basic mechanical design.

You do all the work to shift gears, don’t you?

But what you don’t see are all the parts that are constantly in motion to keep your car under your control.

One of those parts is the flywheel. In a vehicle with a manual transmission, the flywheel serves a crucial purpose.

Let’s take a closer look at what a flywheel is, what it does, and what could go wrong.


In a manual transmission, the flywheel is a thick metal disc.

It’s typically made of cast iron, steel or, in some cases, aluminium.

It’s extremely rigid to prevent flexing or warpage during use.

The edge of the flywheel has a row of gear teeth that engage with the engine’s starter motor.

The flywheel is firmly bolted to a flange on the transmission side of the crankshaft inside the bell housing.

On the side facing the manual transmission, the surface is machined flat for the clutch disc to grab onto.


But what does the flywheel do? It actually has a few different purposes:

  • The flywheel provides mass for rotational inertia to keep your car’s engine running. Otherwise, the engine will stall when you let your foot off the accelerator.
  • It balances the engine. A flywheel is specifically weighted to the car’s crankshaft to smooth out the rough feeling caused by even a slight imbalance.
  • It allows for an electric starter. The starter motor engages the starter ring on the edge of the flywheel to begin engine rotation.
  • Most importantly for drivers, the flywheel connects the engine with the transmission via a clutch to transfer power to the wheels.

When your foot is on the clutch pedal, the clutch disc is disengaged from the flywheel.

This is how a car can sit still at idle with the shifter in gear, or how a car can coast to a stop.

But when the pedal is released, the clutch disc will press firmly against the flywheel.

When this happens, the transmission input shaft rotates at the same speed as the engine’s crankshaft.


Due to its heavy-duty construction, a flywheel is extremely durable.

That’s not to say that problems can’t occur, just that they’re less frequent.

Among the most common flywheel problems that can come up is contamination.

A leaking rear main crankshaft seal or a transmission front input shaft seal can leak oil onto the flywheel.

It causes clutch slippage and excessive friction and heat.

Warpage or hot spots can result, creating a vibration on engagement and acceleration.

In extremely rare cases, a flywheel can crack due to hard use in high-performance or heavy-duty vehicles.


Sometimes, the flywheel can be resurfaced to restore a flat, even surface for the clutch to mate to if the concern is warpage or hot spots.

If resurfacing the flywheel isn’t possible, replacement is the only other repair.

Resurfacing a flywheel is, on average, around $500 to $650 for the labour involved.

If a replacement flywheel is required, the average repair cost is $700 to $1,200 depending on the make and model.

It’s also a great time to change the clutch if it’s nearing time to replace it.

If you need your Flywheel replaces, you can find quality, local mechanics on AutoGuru.

The best bit? You can get quick quotes and book it all in online!

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Written By

Jason Unrau

Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.

One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.

Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.