Set of 4 Both sides are of flat strong steel and in between rubber to absorb all the vibrations. One end has two mounting 8 mm holes on the other end 8 mm fixed bolt provided for perfect installation. It is designed to provide maximum control of all engine harmonic vibrations, regardless of the rpm at which they occur and regardless of the crankshaft's natural frequency. This translates to maximum protection of the crank, main bearings, related engine components and more consistent power output. It is maintenance and repair free.