A harmonic balancer is a device that is mounted onto the crankshaft of an internal combustion engine. The harmonic balancer is a pulley-like device comprised of two cast-iron components coupled by a rubber ring. As the engine spins, harmonic vibrations from the valve train as well as the entire rotating assembly — which would otherwise destroy the engine's main bearings — are absorbed by the rubber ring and balanced by the outer iron part of the harmonic balancer. Most timing marks that are used to set ignition timing on a vehicle are also scribed onto the outside edge of the harmonic balancer.
In a passenger vehicle, the stock harmonic balancer is usually sufficient. It is designed to last the duration of the vehicle's engine, and in most applications, it does just that. Oil that may leak onto the balancer, however, can deteriorate the rubber inner component of the harmonic balancer and cause it to break loose. When this happens, the vehicle may run noticeably rough or not run at all. The trouble can be caused when manufacturers mount the ignition trigger on the harmonic balancer.
In a high-performance application, the stock design of the balancer will not sufficiently protect the engine. The high-speed operation of a racing engine would cause the balancer to separate at the rubber ring and thereby break the engine. Occasionally, a stock harmonic balancer would break and actually be thrown through the vehicle's hood. This was a very dangerous occurrence and led to stock balancers being prohibited for use in racing applications. Racing balancers are constructed of a solid steel outer shell and are filled with a thick viscous fluid inside to absorb the harmful vibration.
Unlike the stock units, racing type balancers are marked with a special date code and require replacement after a predetermined time period. It is up to the individual race-sanctioning bodies to set the replacement date and perform the safety inspections. When installing a high-performance balancer on an engine, it is typically suggested that a high tensile strength bolt be used to secure the unit to the engine's crankshaft. With only a single bolt affixing the balancer to the crankshaft, this is a wise rule of thumb to which to adhere. Many companies manufacture special high-strength bolts for this particular application, and a balancer bolt should never be reused once removed from an engine.