Patent ID: 8221274

Claim:
A chain tensioner comprising: a cylinder having a closed end; a plunger axially slidably inserted in said cylinder for pressing a chain, said plunger defining in said cylinder a pressure chamber, to be filled with hydraulic oil, such that a volume of said pressure chamber is variable with axial movement of said plunger, said pressure chamber communicating with an outside of said cylinder through a restricted passage defined between sliding surfaces of said plunger and said cylinder; a spring biasing said plunger in a direction to increase the volume of said pressure chamber, said cylinder being formed with an oil supply passage through which said pressure chamber communicates with the outside of said cylinder; and a check valve which allows only a flow of hydraulic oil through said oil supply passage from the outside of said cylinder to said pressure chamber; wherein said check valve comprises an annular valve seat, a check ball which is movable into and out of contact with said valve seat, and a retainer retaining said check ball and being formed by drawing; wherein said retainer comprises a tubular portion and a bottom at a first end of said tubular portion; wherein said tubular portion of said retainer is formed with an opening, which communicates between an interior and an exterior of said retainer, through which hydraulic oil can flow into and out of said retainer; and wherein said opening extends to a junction of said tubular portion and said bottom of said retainer and is formed by punching said tubular portion from inside to outside thereof such that said opening is defined by a punched edge surface including a shear surface portion and a ruptured surface portion located outwardly of said shear surface portion, said ruptured surface portion being rougher than said shear surface portion.