Lubricating arrangement in valve mechanism of an overhead camshaft engine

In a valve mechanism for an internal combustion engine of the overhead camshaft type, including a cylinder head having a lubricating oil passage formed therein, at least one pair of intake valve and exhaust valve, a pair of rocker shafts, each having a substantially cylindrical outer configuration and a hollow interior defined therein, each of said rocker shafts having a peripheral wall thereof formed with a pair of diametrically opposite through holes, at least one pair of rocker arms supported on said rocker shafts for rocking motions thereabout and drivingly coupled, respectively, to said intake valve and said exhaust valve, a camshaft drivingly coupled to said rockerarms for causing rocking motions thereof, whereby rocking motions of said rocker arms in response to rotation of said camshaft cause said intake valve and exhaust valve to be closed and opened alternately, a camshaft holder rotatably supporting said camshaft and supporting said rocker shafts, said camshaft holder having first and second bolt-fitting holes formed therein, and first and second bolts extending through respective ones of said through holes of said rocker shafts and said first and second bolt-fitting holes of said camshaft holder and threadedly fitted in said cylinder head to fasten said rocker shafts and said camshaft holder thereto, the improvement wherein said camshaft holder has first and second bearing recesses, each having a generally semicylindrical shape corresponding to said outer configuration of said rocker shafts, said first and second bearing recesses opening in a side surface of said camshaft holder remote from said cylinder head and arranged in spaced relation to each other, said rocker shafts having part of said peripheral walls thereof supportedly received in respective ones of said first and second bearing recesses, a first lubricating oil passage formed in said camshaft holder and having one end communicating with said lubricating oil passage formed in said cylinder head and another end opening in one of said first and second bearing recesses, one of said rocker shafts being received in said one bearing recess and having a second lubricating oil passage formed therein and communicating said first lubricating oil passage with said hollow interior of said one rocker shaft, said first and second bolt-fitting holes extending from respective ones of said first and second bearing recesses to another side surface of said camshaft holder facing said cylinder head, one of said through holes of each said rocker shafts communicating said hollow interior of said each rocker shaft with a corresponding one of said first and second bolt-fitting holes, a corresponding one of said first and second bolts extending through said one through hole, said first and second bolt-fitting holes and said one through holes of said rocker shafts each having an inner diameter larger than the outer diameter of said corresponding one bolt to define first and second annular gaps, respectively, between said first and second bolts and corresponding ones of said bolt-fitting holes and corresponding ones of said one through holes, said camshaft holder further having a recess formed in said another side surface thereof and having an inner peripheral surface, said recess cooperating with a recess formed in a surface of said cylinder head facing said camshaft holder to define a generally cylindrical bearing space in which said camshaft is supportedly fitted, a circumferentially extending peripheral groove formed in said inner peripheral surface of said recess of said camshaft holder, and first and second communication passages formed in said camshaft holder and communicating said peripheral groove with respective ones of said first and second annular gaps.