There comes a time in the life of every flywheel when it needs to be resurfaced. That time is usually when the clutch is replaced. Normal clutch operation generates a lot of friction and heat. The mass of the flywheel absorbs and dissipates the heat. If the clutch is starting to slip, even more heat is generated. The added thermal stress can cause heat cracks, warpage and the formation of hard spots in the surface of the flywheel.
When the old clutch is removed, the flywheel should always be carefully inspected to determine its condition. This includes measuring the flatness of the flywheel with a straightedge and feeler gauge, and inspect the surface for cracks, grooving or hard spots (discolored areas that are slightly raised above the surrounding surface).
Some specifications allow a maximum runout of up to .0005 inch per inch of flywheel diameter. But according to some experts, more than .002 inches of runout on a typical passenger car flywheel may cause chatter and vibration problems. More than .005 inches of runout increases the risk of severe vibrations that may cause the clutch to fail. Flatter is always better.
Of course, if the surface of the flywheel is flat and free from defects, there is no need to resurface it. But if it is not in like-new condition, the flywheel should be resurfaced before the new clutch is installed.
If a worn flywheel is not resurfaced, the replacement clutch won't last. Most clutch suppliers will not honor such a warranty claim if the flywheel was not resurfaced (or was resurfaced incorrectly) when the clutch was installed. Installing a new clutch disc on a worn or warped surface is asking for trouble, yet all too often the flywheel is not resurfaced to save time or money.
If a flywheel is found to be damaged (cracks that are more than surface deep, or cracks around the crankshaft bolt holes), replacement is required. A cracked flywheel can explode with tremendous force, so under no circumstances should you take a chance on a flywheel that is questionable.
On many flywheels, the starter ring gear is a separate component that is pressed on ó and can be replaced if any of the teeth are damaged. If the teeth are part of the flywheel itself and are damaged, a new flywheel should be installed to eliminate any possible cranking problems.
If a flywheel needs to be resurfaced or replaced, its index position with respect to the crankshaft should be clearly marked prior to removal to maintain proper engine balance. This step is critical with engines that are "externally" balanced (those that donít have large flywheel counterweights and rely on the balance of the flywheel to minimize vibrations).
With dual-mass flywheels, resurfacing is not recommended on BMW, General Motors or Porsche. If a dual-mass flywheel on one of these vehicles is worn, it must be replaced.
On Ford applications, a dual-mass flywheel can be resurfaced by removing the bolts and separating the primary and secondary flywheels. Ford recommends using new bolts when the flywheel is reassembled.
Conventional one-piece flywheels are available to replace the more expensive dual-mass flywheels on some applications such as 7.3L Ford and 6.5L GM diesel trucks. The solid flywheels do not use the same clutch as the original, and some are designed for a larger diameter clutch to beef up the torque capacity of the drivetrain. A solid flywheel will not provide the same smoothness or vibration dampening characteristics of a dual-mass flywheel, but they are more reliable and affordable.
Flywheels can be resurfaced two ways: by cutting or grinding. Cutting is usually done on a brake lathe. Setting up a flywheel on a lathe takes times and must be done carefully to make sure the flywheel turns true on the lathe. One drawback with cutting is that a lathe bit tends to skip over hard spots, leaving uneven areas.
The alternative is to remove a greater amount of metal, which may have an adverse effect on installed clutch height. On vehicles with hydraulic linkages, the release bearing may have limited travel. If too much metal is removed from the flywheel, the clutch may not fully release if the hydraulic linkage is at the limit of its travel.
Grinding is still the preferred method for resurfacing most flywheels. Grinding can be done on a head and block grinding machine, or a dedicated flywheel grinder. Grinding equipment designed for heads and blocks, though, can only handle flat flywheels and takes longer to setup than a dedicated flywheel grinder. If a stepped or recessed flywheel needs to be ground, a dedicated flywheel grinder designed for this purpose must be used. If a machine shop does not have this type of equipment, find one that does.
On applications where a stepped flywheel is used (Honda and VW, for example), equal amounts of metal must be shaved off of both surfaces to maintain the proper clutch height and pressure. In other words, if .010 is removed from the lower step, .010 must also be removed from the upper step to maintain the same relationship. This requires using a flywheel depth gauge to measure the amount of recess before and after resurfacing.
A dedicated flywheel grinder with an overhead stone rotates the flywheel while it is being ground to achieve the required flatness with minimal metal removal. A grinder will remove hard spots and leave a smooth, homogeneous surface. Grinding time is typically three to four minutes. The flywheel is mounted using the crankshaft flange as a reference point, and custom adapters or centering cones can be used to center a recessed flywheel.
The proper surface finish can be achieved by wet grinding with silicone carbide stones or dry grinding with CBN stones (the latter are more expensive, but longer lived). Softer stones are recommended for grinding forged steel flywheels, while hard stones work best on cast iron flywheels. Using the proper coolant is important for long stone life and good cutting action. Water-based coolants should also contain a rust inhibitor to prevent rust spots from forming on a resurfaced flywheel.
When a recessed flywheel is ground, the stones leave a radius on the corner of the clutch cover mounting surface, whether the step is internal or external. This radius should be removed so the clutch pressure plate will match up squarely when it is installed.
When replacing a clutch, remember the clutch is a system so all the major parts should be replaced as a set. This includes the clutch disc, pressure plate and release-bearing. If the vehicle has a pilot bearing or bushing, this should also be replaced. The pilot bushing supports the end of the transmission or transaxle input shaft and aligns the clutch disc to the flywheel. If worn, it can cause rapid clutch and throw-out bearing wear as well as clutch engagement/disengagement problems.
If you are replacing a flywheel on an engine that is internally balanced, the flywheel and damper have no effect on engine balance and can be removed without indexing the flywheel's position on the crank. But if the engine is externally balanced, the index position of the flywheel must be marked BEFORE you remove it so it can be installed back on the crank in the same position as before. This is necessary to maintain proper engine balance. If you don't know if the engine is internally or externally balanced, play it safe and mark the flywheel before you remove it. Then install it in the same position as before after it has been resurfaced.