Resurfacing a flywheelJune 26, 2018
A common mistake that both farmers and professional mechanics alike make is not resurfacing the flywheel when a clutch is replaced.
Though the flywheel is made of heavy-duty material, it is exposed to friction heat even when the clutch disc and pressure plate are not slipping. This thermal cycling over time will degrade, warp and change the surface metallurgy (hard spots). When this occurs, the clutch has the potential to chatter, vibrate, grab, and stick, along with premature failure of the new disc and pressure plate. The clutch is designed to work on a parallel surface.
A worn, cupped or crooked flywheel can also cause release problems, especially in an application that does not have a good deal of inherent travel.
If the clutch was slipping prior to being renewed, an excessive amount of heat was induced to the flywheel, and all areas of concern are amplified.
A new clutch assembly that is mated to a flywheel that was not surfaced will have a dramatic reduction in service life. To access most clutch assemblies the tractor often needs to be split, not a task that you want to do twice.
There are two prescribed methods to refinish a flywheel: turning on a lathe or grinding with an application specific machine. Many try to turn a flywheel on a brake lathe but the industry frowns on that procedure. It is very difficult to accurately chuck the flywheel to a brake lathe. Often, any heat induced spots cannot be removed. For this reason, always ask the machine shop if they use a flywheel grinder before evoking their services.
Determining if it needs to be surfaced
If the clutch is worn then the flywheel needs to be resurfaced; it is that simple. The proper method would be to take measurements with a dial indicator and magnetic base while the flywheel is still attached to the crankshaft; the pragmatic being, if the disc is worn the flywheel is distorted.
The action of the clutch can and will cause the following to a flywheel:
Face run out: This is the variation or wobble of the flywheel surface that the clutch disc marries to. The SAE J6186 standard for run-out is a maximum of 0.0005-inch (5/10,000 inch) inch per of flywheel diameter. I have seen issues occur in automotive applications with as little as 0.0002-inch per flywheel diameter.
Excessive flywheel run-out is usually the result of previously misaligned resurfacing methods such as on a brake lathe or sloppy fixturing to a flywheel grinder.
Dishing: Dishing is less common with cast iron flywheels than steel. It is a pocket that can be as deep as 0.010 inch and not detected by the naked eye. The only way is with measurement.
Hard spots and discoloration: Excessive clutch slippage will create nucleation and growth of carbide hard spots and will need to be ground out.
Groove wear: This can be seen and felt during inspection and will be found on the flywheel friction surface along with the clutch disc.
Ring gear wear: Inspect the condition of the ring gear. Keep in mind that worn teeth represent that weight has been lost and the balance of the engine will be impacted.
A time for new
Proper inspection and measurement of a flywheel by a professional will determine if it can be put back into service. Much like a brake rotor or drum, it will need to be ground to see if the damage can be removed while leaving enough mass for it to work properly. I always like to ask the machine shop how much material they removed so I can determine the degradation based on my thoughts.
When I purchase a new flywheel I bring it to the machine shop and have it set up on the grinder and dressed to make sure that it is true. Do not assume because it is new that is the case. The market is rife with poor quality imports and rough handling during shipment.
I was led to believe by my local auto rebuilder in the 60’s (old school) that a flywheel was to be surfaced with a slight concave dish to eliminate chatter and make for a smoother engagement. Those were single-discs, everyday, mom & pop grocery-getters. Does this still hold true for the old nostalgic vehicles one might be restoring in this day and age?
Thank You Wayne