Many hot rodders buy a quality rotating assembly then go cheap on a balancer. However, keeping crankshaft harmonics under control is critical to engine longevity. The combustion process sends vibrations and pulses through the crankshaft on every power stroke. At certain rpm, these frequencies can resonate and the resulting harmonics can rattle an engine apart. Anything you can do to reduce harmonics is beneficial. The most obvious solution is using a high-quality harmonic damper. For high-rpm race motors where regulating harmonics are more important, engine builders can change the firing order around. While this does not eliminate harmonics, it spreads them out more evenly. Furthermore, you can also control harmonics by balancing a crank differently. If you know the rpm range of the motor, you can over- or under-balance a crank. This involves adding or removing weight from the bob weight during balancing to simulate the bending loads placed on a crankshaft.
Minimum Rod Length
When piecing a rotating assembly together out of a catalog or off the Internet, there are some important pitfalls to avoid. Crankshaft catalogs are often published with a minimum connecting rod length. Some hot rodders don’t see how a crankshaft would affect rod selection, so they ignore this specification only to pay the price later. However, this is a very important specification because it tells you how long a rod must be to clear the piston. For instance, if you have a 3.750-inch crank listed with a 6-inch minimum rod length, that means the crankshaft counterweights will clear the pistons with rods that are 6 inches or longer. If you use the same crank but with 5.700-inch rods, the crank will still work but the counterweights will hit the pistons.
If you have access to a large lathe, another option is cutting down the counterweights for additional clearance. If you have a crank with a 6-inch minimum rod length, but want to use a 5.850-inch rod, cutting the counterweights down 0.15 inch to make it work isn’t a problem. Lunati offers these services in-house if a customer lets us know what they want when placing their order. Keep in mind that the purpose of the counterweights is to counter the mass of the rods and pistons, so the more metal you remove from them, the more Mallory you may have to add back to crank to get the rotating assembly balanced. However, if you have super-lightweight rods and pistons, then it might not be necessary to use heavy metal in the crank.