http://www.aa1car.com/library/2003/bf100372.htm-Larry Carley said:
RESURFACING TECHNIQUES
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 the preferred method for resurfacing most flywheels today. 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.
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.