An upper or Cam to Crank Timing Chain synchronizes the rotation of the crankshaft and the camshafts. It is recommended to replace this Chain when Replacing the Timing Chain Tensioner and Guide Rails. While Timing Chains are designed to last a lifetime, it is possible for them to stretch and possible for the tensioners to fail. We recommend inspecting these components to prevent severe engine damage, especially on high mileage vehicles (over 150K).