| 摘要 |
A valve timing control system has a tubular housing; a cam shaft having an external periphery formed with a drive cam; 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. The tubular housing has a housing body having a density, and a sprocket portion for receiving a drive force transmitted from a crank shaft of an engine by way of a chain. The sprocket portion is disposed integrally to the tubular housing, and has a density higher than the density of the housing body. The tubular housing is so mounted to the cam shaft as to make a rotation relative to the cam shaft when so required.
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