Patent ID: 7331891

Claim:
A chain or belt tensioner comprising a housing and a plunger displaceable in a bore of the housing between retracted and extended positions, the plunger having a first end disposed in said housing and a second end for projection from said housing, a hollow area defined in said plunger, a variable volume chamber defined by the hollow in said plunger and the bore for receipt of hydraulic fluid, a source of hydraulic fluid, a check valve disposed to allow transfer of the hydraulic fluid from the source to the chamber when the fluid reaches a predetermined pressure and to prevent flow of hydraulic fluid in the reverse direction, biasing means for biasing the plunger towards the extended position, characterised in that an outside surface of the plunger forms a fluid tight seal with an inside surface of the housing bore and in that there is provided a passage defined on one of the outside surface of the plunger and the inside surface of the housing, the passage extending along a length of the bore between the plunger and the housing and having at least one inlet end adjacent to and in communication with said chamber and an outlet end in communication with atmosphere, the passage permitting the purging of air trapped in the chamber; wherein the passage has two substantially circumferential portions intermediate said inlet and outlet ends and which are spaced apart and joined by an axially extending portion of the passage.