Tensioner

A tensioner comprising:

• a plunger having a plunger hole that is open on a rear side;
• a housing having a housing hole that is open on a front side and accommodates the plunger;
• an oil pressure chamber formed between the plunger and the housing hole;
• a main biasing unit that biases the plunger toward the front side;
• a resilient ring having a ring-like part that is disposed on an outer circumference of the plunger and engages with an engaging groove formed on an outer circumferential surface of the plunger;
• and a ring control mechanism that controls a position of the resilient ring,
• the ring control mechanism including a front-side restricting part having a front-side restricting surface disposed on the front side of the ring-like part and capable of restricting forward movement of the resilient ring, and a rear-side restriction member having a rear-side restricting surface disposed on the rear side of the ring-like part and capable of restricting rearward movement of the resilient ring,
• the front-side restricting part being fixed to or formed integrally with the housing, and
• the rear-side restriction member being disposed movable in a front to back direction relative to the housing and the plunger.