The variable cam timing unit assembly (Figure 131) is coupled to the camshaft through a helical spline in the VCT unit chamber. When the flow of oil is shifted from one side of the chamber to the other, the differential change in oil pressure forces the piston to move linearly along the axis of the camshaft. This linear motion is translated into rotational camshaft motion through the helical spline coupling. A spring installed in the chamber is designed to hold the camshaft in the minimum overlap position (5 degrees) when oil pressure is too low to maintain adequate position control. The camshaft is allowed to rotate up to 30 degrees .