This hexagon head bolt secures the tensioner pulley to its mounting bracket. It passes through the pulley arm and threads into the bracket, maintaining the correct belt tension by holding the pulley in position. The bolt is manufactured to the hardness class required for the clamping load needed in this application. If the bolt is reused without inspection, thread stretch or fatigue can allow the pulley to shift, causing belt slip or misalignment. On reinstallation, replace the bolt if any deformation is visible and tighten to the specified torque.