Torsional Vibration Damper (TVD) for the DD15 platform. 

A new Damper will prolong your engine life by eliminating harmful torsional crankshaft vibration. OEM manufacturers recommend replacing the damper at 500,000 miles. Failure to replace a worn damper could result in a damaged crankshaft and other serious engine failures. Keep your truck out of the shop with this OEM, torsional damper.