Valve timing control system and method of producing valve timing control system

A valve timing control system comprising:

• a tubular housing which is a sintered body, comprising:
• a housing body having a density,
• a sprocket portion for receiving a drive force transmitted from a crank shaft of an engine by way of a chain, the sprocket portion being disposed integrally to the tubular housing and being formed substantially in an axial center of the housing body, the sprocket portion having a density higher than the density of the housing body;
• a cam shaft having an external periphery formed with a drive cam for operating an engine valve, the tubular housing being mounted to the cam shaft and making a rotation relative to the cam shaft when so required, the cam shaft receiving the drive force transmitted from the sprocket portion, to thereby rotate as a follower;
• a phase variation mechanism disposed in the tubular housing, and varying a rotational phase of the sprocket portion relative to the cam shaft in accordance with oil pressure supplied to the phase variation mechanism;
• and
• an oil pressure control measures for controlling the oil pressure supplied to the phase variation mechanism.