Weight Reduction

Cranks often have lightening holes drilled into them to reduce mass, but whether or not this technique is effective depends on how the lightening is performed. Gun drilling the main journals does reduce mass, but only from the centerline area of the crank. Since the rotating weight of the crank remains the same, there are no horsepower benefits to this type of drilling. On the other hand, drilling lightening holes in the rod journals does indeed remove rotating weight, so it does offer some potential horsepower benefits. A common misconception is that since removing rotating weight is beneficial, cutting down the counterweights can also be beneficial. The truth is that cutting holes into the counterweights is a hack job and can wreak havoc on the balancing process. Since the counterweights offset the mass of the pistons and rods, any weight that’s removed from the counterweights will usually have to get added back in with slugs of Mallory. If you can get away with trimming down the mass of the counterweights, it’s much better to grind them down on a big lathe instead of drilling holes into them. Every hole that’s drilled into the counterweights increases drag as they move through the oil. That’s why very high-end racing cranks have no holes drilled into the counterweights at all.

Machining

During the manufacturing process of a crankshaft, it must be transformed from a raw forging into a precise piece of hardware that has very tight tolerances. Although forgings are far stronger than castings, a drawback of the forging process is that it requires a lot of finish machining. Many people would be shocked if they saw what a raw crank forging looked like, because it has lots of extra material on it and doesn’t really resemble a finished crank at all. As such, Lunati has many quality control procedures in place to ensure that tolerances in critical areas, like the journals and bearing surfaces, are kept as precise as possible. The biggest key to precise machining is establishing the centers since everything is based off the crankshaft centerline. Once the centers are established, the counterweights are machined and brought to a rough grind. Most of the process is performed on CNC machines, but the finishing grinding of the main and rod journals are still done manually by hand. Some might ask why you would manually grind the crank journals. The answer is we’ve had cranks that needed two tenths of a thousandth of an inch removed off a crank for clearance, and our guys can do it. To give you an idea of how much labor is involved in building one crank, it takes 30 hours from start to finish. If one person had to walk a crank through the manufacturing process himself it would take four days.