Chain tensioner

A chain tensioner comprising a housing defining a cylinder chamber having an opening at a front end of said housing and configured to be mounted on an engine block, a plunger slidably mounted in said cylinder chamber, a spring mounted in said cylinder chamber and biasing said plunger out of said cylinder chamber through said opening, and a resiliently and radially deformable register ring received in a ring-receiving groove formed in an inner periphery of said cylinder chamber near said opening, said plunger having a plurality of annular grooves formed in an outer periphery thereof so as to be axially spaced apart from each other at equal intervals, each of said annular grooves comprising a tapered surface of which the diameter gradually decreases toward the front end of said plunger, and an engaging surface extending from the small-diameter front end of said tapered surface toward the front end of said plunger, said ring-receiving groove including a locking surface at a rear portion thereof, said locking surface and said engaging surface of any of said annular grooves being capable of trapping said register ring therebetween, thereby preventing said plunger from retracting into said cylinder chamber, said plurality of annular grooves including a first annular groove located nearest to the front end of said plunger, said plunger having a set groove formed in an outer periphery thereof at its portion displaced from said first annular groove toward a front end of said plunger, said set groove having a depth smaller than the wire diameter of said register ring and being capable of retaining said register ring therein, an outer diameter of said plunger other than where said set groove and said annular grooves are formed being constant over an entire axial length of the outer cylindrical surface of said plunger, said plunger being further formed with an outer cylindrical surface extending from said set groove toward the front end of said plunger, said outer cylindrical surface having an axial length determined such that, with said register ring trapped between said engaging surface of said first annular groove and said locking surface of said ring-receiving groove, said outer cylindrical surface is slidably supported by an inner cylindrical surface formed on an inner periphery of said cylinder chamber to extend from said ring-receiving groove toward the front opening of said housing, said set groove having an axial width that is smaller than the axial length of said inner cylindrical surface.