The harmonic balancer is made up from three parts: Outer pulley, inner damper and a rubber gasket.

At the factory, the gasket is placed between the pulley and damper, then the pulley/gasket/damper is pressed together forming a one piece assembly.

When the gasket age cracks, shrinks, this causes the outer pulley to "walk away" from the inner damper.

If not caught in time, the parts will separate, the pulley may strike the radiator core, taking associated parts along with it. Not pleasant.