When it comes to cheap performance modifications, you can’t do better than to lighten your engine’s flywheel. Reducing your engine’s MOI will get you the most noticeable bang for your buck. The best way is with a lightened flywheel. Unfortunately, a simple task like machining a flywheel can quickly become complicated and is often done incorrectly. Some important lightening details are as follows:

Remove material where it affects the moment of inertia (MOI) the most. Removing material near the crank hub is unnecessary and likely dangerous. This area of the flywheel is under the greatest stress, why remove material from there and risk the part failing? Removing material near the ring gear is best and will give you the greatest effect.

Generous radii, no sharp corners. Corners are stress risers and you want to minimize stress concentrations. Doing so will result in a stronger, more robust part.

Flywheel material composition. Beware that cast flywheels typically have voids in them. This is why minimum thickness rules cannot be applied across all different flywheels. Never say “I’ve seen this done on other flywheels,” every flywheel design and manufacture should be evaluated individually. The material quality is vastly different between Fiat and BMW flywheels, even between different model flywheels of the same company.

Balance. After any lightening procedure the flywheel should be dynamically balanced (aka zero balance). Even though a flywheel is lightened on a lathe and is dimensionally symmetrical, this does not imply that it is balanced. Cast materials are not a consistent density throughout and will affect the balance. Preferably, flywheels should be balanced with the whole rotating assembly (crankshaft, crank pulley, flywheel, and clutch) but this is only possible if the engine is disassembled.

Grind friction surface. It may be hard to believe but after you lighten a flywheel, it is possible for the whole part to warp due to the removal of material and internal stresses. This can make the friction surface uneven and it should be checked and reground. Sometimes the friction surface does not warp and this is not a necessary step. Do not “turn” the friction surface on a lathe. The proper procedure is to blanchard grind it. Turning creates tiny grooves parallel to the clutch’s direction of travel, this makes it difficult for the clutch material to “bed-in” to the flywheel and create friction. A blanchard grind, however, makes lines perpendicular to the direction of travel, this encourages “bed-in” and friction, which is what we want! In addition, turning will not be consistent over hard spots. See the picture below. The shiny spots are hard spots, and the dull spots are the normal material. All hard spots must be removed, because they will cause uneven ware and are very slippery (we don’t want slippage). All cracks must also be removed.

Check for bolt embedding: Loose flywheel to crank bolts can create all kinds of havoc. One result is that the bold heads can “embed” themselves into the flywheel material, resulting in a surface that is no longer smooth and ideal for carrying the bolt’s load. The solution is to “clean-up” the surface by skimming with a lathe or grinder to make it smooth again.

Detail work: When reconditioning a flywheel it is nice to see the other small details like the clutch mounting threads nice and clean, sharp edges deburred.  A dye penetrant or magnaflux check for cracks is also recommended for peace of mind.

We can evaluate any flywheel for lightening and race preparation. Our prices are as follows:

Evaluate crack check, clean: $40
Flywheel lightening: $60
Balance (flywheel only): $40
Regrind friction surface: $40
Fiat dowel pinning: $30

We can also supply the appropriate ARP mounting hardware for flywheels and clutches. See parts pages for more details.