A harmonic balancer, also called a crankshaft damper or vibration damper, bolts to the front of the crankshaft and absorbs the twisting vibration the crankshaft picks up every time a cylinder fires. Without it, that vibration can build at certain engine speeds and crack the crankshaft or damage bearings. On most engines the balancer also carries the pulley for the accessory belt and the timing marks.
What does a harmonic balancer do?
Each power stroke pushes on the crankshaft for a moment, so the crank does not turn perfectly smoothly: it winds up and springs back very slightly, many times per revolution. This is called torsional vibration. At some engine speeds those pulses line up with the crankshaft’s natural frequency and the twisting grows, which loads the crank, its bearings and anything driven from it. The balancer is built to soak up that energy before it reaches a damaging level.
How a harmonic balancer works
- Elastomer dampers: the most common factory design. An outer inertia ring is bonded to an inner hub with a rubber layer. The ring lags slightly behind the crank’s vibration, and the rubber turns that movement into a little heat.
- Fluid (viscous) dampers: an inertia ring floats in a thick silicone fluid inside a sealed housing, and the fluid absorbs the vibration across a wide speed range. Fluidampr is a well-known example; the D&J Ready Run 6.7L Cummins engine package we carry lists a Fluidampr harmonic balancer among its parts.
Despite the name, a harmonic balancer is a damper rather than a counterweight. On some engines it is also part of the external balance, so it must match the engine it goes on.
Symptoms of a bad harmonic balancer
- Wobble: the pulley or outer ring visibly wobbles with the engine running.
- Belt trouble: the accessory belt squeals, wears on one edge or walks off the pulleys.
- Damaged rubber: the rubber between hub and ring is cracked, missing or bulging out of the gap.
- Timing marks that do not line up: a sign the outer ring has slipped on the hub, which also makes the marks useless for checking timing.
- New vibration: vibration or roughness at particular engine speeds that was not there before.
- Noise: knocking or rattling from the front of the engine, which can also come from a loose balancer or crank bolt.
Do not keep driving with a balancer that is coming apart. If the outer ring separates, it can take out the belt and damage parts at the front of the engine.
Replacing a harmonic balancer
- Remove the accessory belt and anything in the way, such as the fan shroud.
- Hold the crankshaft and remove the crank bolt.
- Pull the balancer off with a proper balancer puller that threads into the hub; never pry on the outer ring.
- Inspect the crank snout, the key and the front seal, and replace the seal if it leaks or the old balancer wore a groove into the seal surface.
- Press the new balancer on with an installer tool rather than hammering it, which can damage the balancer and the crankshaft thrust bearing.
- Fit the crank bolt and tighten it to the specification in the service manual. Some engines use a single-use bolt that must be replaced.
Overdrive dampers on supercharged engines
On belt-driven supercharger kits, the crank pulley size helps set how fast the supercharger spins. An overdrive damper has a larger pulley diameter, so it spins the supercharger faster and raises boost. The Innovators West crank damper we sell for the East Coast Supercharging direct drive setup is offered in 10% or 15% overdrive and has a cog drive provision; it is $1,100.00 on its own and already comes with the ECS direct drive system. More boost needs the fuel system and engine calibration to match, so plan the whole combination.
Related parts and guides
See the Innovators West crank damper, our supercharger kits by platform and the D&J Ready Run 6.7L Cummins engine. Fitting a new or rebuilt engine? Follow an engine break-in procedure and check the firing order before the first start.
More guides: what does a crankshaft do?.
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