Harmonic Balancer
A heavy metal pulley with a rubber damping ring bolted to the front of the crankshaft to reduce engine vibration and drive the serpentine belt.
The harmonic balancer (also known as a crankshaft damper) is a two-piece metal pulley bolted to the front nose of the crankshaft. It consists of an inner hub and an outer pulley ring connected by a thin band of elastomeric rubber.
As the cylinders fire, the force of combustion twists the crankshaft slightly, causing torsional vibrations that can fatigue and break the crankshaft at high speeds. The rubber ring inside the balancer absorbs these torsional shocks, protecting the engine. The outer ring also serves as the drive pulley for the serpentine belt.
Over time, the rubber insert can dry rot, crack, or separate, causing the outer ring to wobble, squeal, slip, or fly off completely, resulting in severe engine damage. Mechanics inspect the harmonic balancer for cracking rubber during routine accessory belt service.