An engine’s rotating assembly is nowhere near “lightweight.” Sure, you can get a “lightweight” crank and rods, but that’s an extremely relative term. The fact is that you have a large mass spinning at significant speeds, and that means that even the slightest imbalance in the assembly will be magnified many times over.
On the minor end of the spectrum, an imbalanced (or improperly balanced) rotating assembly can lead to annoying vibrations felt inside the vehicle. On the opposite end of the spectrum, a significant enough imbalance can lead to high-speed unplanned disassembly of the engine, while it’s still in the engine bay.
Knowing how critical it is to have your rotating assembly balanced, most people undertaking a performance build will accept the extra cost associated with the task and have it done. However, do you know what’s actually happening when the assembly is “balanced”? YouTuber Eric The Car Guy put together a comprehensive video which not only shows the process in detail, but explains what is going on as they balance his Forged 302 Windsor rotating assembly.
Step one of the process is to weigh both the “big end” and “small end” of each individual connecting rod. Each end is moving in a different manner – the big end rotates, and the small end reciprocates – so the weights of the individual ends is important. As Eric points out in the video, with today’s manufacturing processes, getting eight connecting rods to be only a few grams apart from one another, out of the box, is far more common these days, and shooting for the +/- 1 gram variance is an easier task.
Once the rod weight measurements are completed, and the group of rods weight-matched, the whole assembly for one cylinder – connecting rod, wrist pin, piston, and piston rings – is weighed and a little math applied, to create the target weight for the bobweight. The bobweight is a weighted clamp which will simulate the two rod and piston assemblies which will normally ride on each crankshaft journal (in a V8 application). Once the target weight is achieved, the bobweight is attached to the crankshaft before it is spun up to speed on the balancing machine.
In an externally balanced application, the balancer and the flywheel or flexplate needs to be mounted on the crankshaft, as they play an integral part of the balancing equation. Once those are mounted on the crankshaft and the bobweights are affixed to the four crank journals, they can actually spin the assembly up.
One the machine goes through its technical magic, it tells the machinist how much weight needs to be added or removed, and where that weight is needed, in three dimensions. With that information, the machinist can then drill holes to remove weight, or drill holes and press or weld in heavy metal slugs (which are heavier than the removed material) to add weight.
Once the assembly is balanced within 1 gram or better, it is called good, and ready to be installed in the block. While this video is a bit on the lengthy side, it offers a great look inside of a very common process you may or may not be intimately familiar with.