Patent ID: 7559863

Claim:
A chain tensioner comprising a housing defining a cylinder chamber therein, a plunger slidably mounted in said cylinder chamber, defining a pressure chamber in said housing, a return spring mounted in said pressure chamber to bias said plunger outwardly of said cylinder chamber, said housing being formed with an oil supply passage communicating with said pressure chamber, a check valve disposed at one end of said oil supply passage opening to said pressure chamber, said plunger being formed with a rod inserting hole that opens to a rear end surface of said plunger, said rear end surface facing said pressure chamber, a screw rod having an external thread formed on an outer periphery thereof and inserted in said rod inserting hole with said external thread in threaded engagement with an internal thread formed on an inner periphery of said rod inserting hole near the rear end thereof, said external and internal threads having a serration-shaped section, and a spring mounted in said rod inserting hole and biasing said plunger and said screw rod axially away from each other, wherein a stepped set groove is formed in an inner periphery of said cylinder chamber near an open end thereof, said stepped set groove comprising a deep portion and a shallow portion which is located nearer to said open end than is said deep portion, said chain tensioner further comprising an elastically and radially deformable set ring having circumferentially separated ends and received in said set groove in a radially compressed state, said set ring having a pair of radially outwardly extending grip portions provided at the respective circumferentially separated ends thereof and disposed in a cutout formed in a front end portion of said housing, wherein an engaging groove is formed in an outer periphery of said plunger near a front end thereof, said set ring being configured to be engaged in both said shallow portion of said set groove and said engaging groove, thereby retaining said plunger in a position where said plunger is pushed into said housing, with one of said grip portions in abutment with one of a pair of circumferentially opposed end walls of said cutout, thereby preventing said plunger from rotating about its axis in such a direction that an axial gap between said internal and external threads disappears, and wherein when said set ring is engaged in both said shallow portion of said set groove and said engaging groove with said one of said grip portions in abutment with said one of said circumferentially opposed end walls, said axial gap is larger than an axial distance by which said plunger has to be moved to move said set ring from said shallow portion into said deep portion.