Chain tensioner

A chain tensioner comprising:

• a tubular cylinder having an open end and a closed end;
• a tubular plunger axially slidably supported by an inner periphery of the tubular cylinder, the tubular plunger having an open end inserted in the tubular cylinder, and a closed end protruding out of the tubular cylinder;
• a return spring biasing the tubular plunger in a direction in which the tubular plunger protrudes out of the tubular cylinder;
• a sleeve fixedly disposed in the tubular cylinder, with a first end of the sleeve inserted in the tubular plunger and a second end of the sleeve protruding out of the tubular plunger;
• a check valve disposed at the first end of the sleeve, the check valve being configured to allow only a flow of hydraulic oil from inside the sleeve toward outside of the sleeve;
• a pressure chamber defined in the tubular plunger such that a volume of the pressure chamber changes with axial movement of the tubular plunger;
• a leakage gap defined between an outer periphery of the sleeve and an inner periphery of the tubular plunger such that, when the volume of the pressure chamber decreases, hydraulic oil leaks from the pressure chamber through the leakage gap;
• a cylindrical space defined between the outer periphery of the sleeve and the inner periphery of the tubular cylinder, the cylindrical space being defined by the tubular plunger such that a volume of the cylindrical space changes with the axial movement of the tubular plunger;
• an oil supply passage defined in the tubular cylinder so as to introduce hydraulic oil from outside the tubular cylinder to inside the tubular cylinder, the oil supply passage opening to the cylindrical space;
• and
• a communication passage defined in the sleeve and providing communication between the cylindrical space and an interior of the sleeve,
• wherein:
• a seating surface is formed on an outer periphery of the tubular cylinder, the seating surface being configured to be fixed to a vertically extending engine wall surface such that the seating surface faces and adjoins the vertically extending engine wall surface;
• and
• the chain tensioner is configured such that, when the seating surface is fixed to the vertically extending engine wall surface, the communication passage is located only at an upper half portion of an entire circumference of the sleeve, and a lower half portion of the entire circumference of the sleeve is free of a communication passage defined in the sleeve and providing communication between the cylindrical space and the interior of the sleeve.