What is Resurfacing?
Through normal use the surface of a flywheel can become worn, pitted or discoloured. Due to the surface being uneven, it doesn’t meet the clutch smoothly which results in clutch slippage or uneven wear and tear.
To remedy this you can remove imperfections by grinding or cutting, resulting in a smooth surface that meets the clutch evenly. Generally you address this when changing the clutch as this process will expose the whole assembly for inspection, allowing you to resurface or replace parts depending on their condition.
When removed you can perform an inspection using a straight edge and feeler gauge to determine its condition.
Why is Resurfacing Performed?
A resurfaced flywheel will have a completely even surface with no flat/hard spots of material. This means the friction across the whole face will be even, making engagement more predictable and smooth. It will also ensure your new clutch is broken in and wears evenly, allowing you to get the full life out of it.
The biggest advantage is more torque can be held by the assembly without slipping, prolonging the life of your clutch. It also makes for smoother and more predictable engagement, making the vehicle easier to drive. As the parts are already accessible when changing the clutch, preventing any premature wear by resurfacing doesn’t involve much more labour or cost.
It’s also a good idea to inspect the flywheel before any resurfacing is performed. Any signs of cracking means the part is unsafe to use and should be replaced immediately. Cracked flywheels can explode whilst in use, which will require a new gearbox as well as serious injury!
On many flywheels the starter ring gear is a separate component, that’s pressed on at the point of manufacture. As it’s a replaceable part ensure that no teeth are damaged. If you can't replace the teeth and there is damage, it's strongly advised to source a replacement.
You will need to consult your vehicle manufacturers tolerances, as material will be ground away to create an even surface you need to ensure this is within manufacturer specification. If there isn't enough material to remove it will need to be replaced.
It’s also important to ensure the machined surface is flat and completely free from defects.
If resurfacing is required it’s a good idea to mark the index position in relation to the crankshaft prior to removal. This is vital with engines that are externally balanced using flywheel counterweights. This step isn’t necessary if the engine is internally balanced.
Dual Mass Flywheels
It’s not recommended to resurface all DMF's specifically from BMW, General Motors or Porsche. If the flywheel on any of these vehicles is worn it should be replaced.
Other manufacturers such as Ford are able to be resurfaced, once removed you need to separate the primary and secondary flywheels and resurface within manufacturer tolerances. All bolts should be replaced prior to replacement.
How to Resurface a Flywheel
The two methods of resurfacing involve cutting or grinding. Cutting will be performed on a brake lathe, care needs to be taken to insure the flywheel turns true on the lathe. This is so that every area of the face is cut evenly to provide a smooth mating surface for the clutch. The cutting of material is performed using a lathe, with the advantage of removing smaller amounts of material compared to grinding. The disadvantage of cutting material away is it doesn’t address hard spots of material, leaving uneven areas.
Grinding is the alternative to cutting, and is generally the preferred method of refurbishing a flywheels surface. Although the disadvantage of grinding is more material is removed, a grinder will remove hard spots of material.
Symptoms of Failure
The most common indicator is vibrations at idle or low speeds. You can also feel the chattering through the pedal, and experience erratic behaviour when the clutch engages. Usually these vibrations are caused by a failed spring mount mechanism of the flywheels, therefore reducing the ability to absorb vibrations from the engine/drivetrain usage.
Another indication of failure is a burning smell in the cabin, some describe the smell as burnt toast. The smell is caused by too much heat on the contact faces, this can also happen due to improper driving. If the smell is only temporary after slippage there shouldn't be any lasting damage, and won't require a replacement.