Expanding piston hydraulic chain tensioner

A tensioner (10) for an endless loop power transmission member can include the assembly of a plunger (12) operably engageable with an endless loop power transmission member, and a piston (14) guiding the plunger (12) for sliding coaxial movement in a direction of the endless loop power transmission member and defining an oil reservoir (26) for receiving hydraulic fluid. A bearing (28) can have bearing balls (30) located in a counter bore (32) coaxial with piston bore (34). The counter bore (32) can have a steep taper (36) on which the bearing balls (30) ride, such that as the plunger (12) moves in an extending direction, the bearing balls (30) move out of the counter bore (32) allowing free extending movement of the plunger (12), and as the plunger (12) moves in a retracting direction, the bearing balls (30) are driven down into the steep taper (36) preventing the plunger (12) from retracting.