TCI Rattler Torsional Vibration Absorber, part # 870001. Harmonic balancer, color black for small block Chevy. 

These Rattler torsional vibration absorbers by TCI are pendulum absorbers developed to control amplitudes of vibration and angle of crankshaft twist like no other design. Their ability to absorb, rather than dampen, is the key to their success. The Rattlers are effective for the entire rpm range and they extend crankshaft and bearing life. The Rattlers also allow the engine to run more smoothly, which can potentially increase valvetrain stability and life. Their design does not utilize viscous fluids and they require virtually no maintenance. You can always depend on the Rattlers' timing marks to be correct because they're etched onto the body and cannot move relative to the crankshaft centerline. The Rattlers are also lighter in weight than most units of the same size and material construction, plus they're SFI 18.1 certified.

Listed in Summitracing.com for $309.97