Resurfacing Flywheels

By Larry Carley

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, thereís no need to resurface it. But if it isnít in like-new condition, the flywheel should be resurfaced before the replacement clutch is installed.

If a worn flywheel is not resurfaced, the replacement clutch wonít last. Your customer will likely be back with a failed clutch disc demanding warranty satisfaction. 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 isnít resurfaced to save time or money. Itís a big mistake that can end up costing a customer a lot more in the long run.

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 anybody take a chance if the flywheel is at all 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 wonít 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 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.

Dedicated flywheel grinders are available from various equipment suppliers. Table diameters range from 12 to 19 inches and can handle flywheels ranging in size from 16 to 25 inches depending on the machine. Some machines are sheet metal and others are cast iron. Some models have no table support while others use roller bearings or an oil bath support. Most use a 1/4 or 1/3 horsepower drive motor for the table and a five horsepower motor for the grinder. Some come with coolant tanks while others do not. So be sure to shop and compare the various grinders before you buy one.

If you do invest in a flywheel grinder, how much you charge to resurface a customerís flywheel will depend on the type of equipment you purchased, your hourly labor rate and local competition. Typical prices charged today for resurfacing flat flywheels ranges from $25 to $48 ó and up to $65 to $100 for stepped flywheels.

Doing the resurfacing work yourself rather than sending the flywheel out to a machine shop can reduce your turn around time while giving you total control over the quality of the work. It will also put more labor profit in your pocket.

According to one equipment supplier, a shop should be able to earn $5,000 extra a year doing just one flywheel a day.

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 the old clutch failed because of abuse, or the vehicle has been modified for more power or is used off-road or for towing, recommend upgrading to a performance style replacement clutch. Performance clutches are designed to handle higher torque loads and can prolong the life of the clutch in applications such as these. But avoid performance clutches that are overly aggressive and sacrifice driveability to achieve more bite.

Another item that should also be replaced is the release cable on older vehicles with this type of linkage. If a vehicle has a hydraulic linkage with a lot of miles on it, it would be wise to replace the master and slave cylinders, too, even if they are not leaking.

Why? Because the slave cylinder is the lowest point in the hydraulic linkage, so most of the rust and sediment that has been accumulating over the years ends up in the slave cylinder. Common sense tells you this will eventually cause problems, so replacing the slave cylinder now will give your customer many more miles of trouble-free driving. At the very least, you should flush the hydraulics and refill the system with fresh fluid.

Balancing goes hand-in-hand with performance clutch and flywheel sets. Balancing reduces internal loads and vibrations that stress metal and may eventually lead to component failure. A smoother-running engine is also a more powerful engine. Less energy is wasted by the crank as it thrashes about in its bearings, which translates into a little more usable power at the flywheel. Reducing engine vibration also reduces stress on motor mounts and external accessories.

No engine is going to survive long at high rpms if itís out of balance. And no engine is going to last in a high-mileage application if the crank is bending and flexing because of static or dynamic imbalances in the flywheel.

To better understand the mechanics of balancing, letís look at the theory behind it. As everybody knows, a rotating object generates "centripetal force."

Centripetal force is an actual force or load generated perpendicular to the direction of rotation. Tie a rope to a brick and twirl it around and youíll feel the pull of centripetal force generated by the "unbalanced" weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple the force.

The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft and flywheel with only two ounce-inches of imbalance at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs.

This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back-and-forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.

Centripetal force should not be confused with "centrifugal" force, which is the tendency of an object to continue in a straight trajectory when released while rotating. Let go of the rope while youíre twirling the brick and the brick will fly off in a straight line (we donít recommend trying this because its difficult to control the trajectory of the brick).

Back to centripetal force. As long as the amount of centripetal force is offset by an equal force in the opposite direction, an object will rotate with no vibration. Tie a brick on each end of a yardstick and you can twirl it like a baton because the weight of one brick balances the other. If weíre talking about a flywheel, the flywheel will spin without wobbling as long as the weight is evenly distributed about the circumference. A heavy spot at any one point, however, will create a vibration because thereís no offsetting weight to balance out the centripetal force.

This brings us to another law of physics. Every object wants to rotate about its own center of gravity. Toss a chunk of irregular shaped metal into the air while giving it a spin and it will automatically rotate about its exact center of gravity. If the chunk of metal happens to be a flywheel, the center of gravity should be the flywheelís axis. As long as the center of gravity for the flywheel and the center of rotation on the crankshaft coincide, the flywheel will spin without vibrating.

But if thereís a heavy spot on the flywheel, or if the flywheel isnít mounted dead center on the crank, the center of gravity and axis of rotation will be misaligned and the resulting imbalance will create a vibration.

If youíre rebuilding an engine that is internally balanced, the flywheel and damper have no effect on engine balance and can be balanced separately. But with externally balanced engines, the flywheel and damper must be mounted on the crank prior to balancing. Owners of externally balanced engines should also be warned about installing different flywheels or harmonic dampers and how it can upset balance.