PURPOSE: To simplify the adjustment of a valve clearance by screwing a valve stem into a valve lifter in such a way as to permit the free advance and retreatment thereof, and hooking a cam carrier to the valve stem for free advance and retreatment and the cam carrier to the valve lifter for turning.
CONSTITUTION: In the adjustment of a valve clearance, the slope 19b of a projection 19a formed on one side of a cam carrier 19 is first lifted and the projection 19a is disengaged from a notch 16c formed on the projected groove 16f of a valve lifter 16. Then, the cam carrier 19 is turned by a predetermined angle and a valve stem 14 with the coupling part 14b thereof engaged with the coupling groove 19d of the cam carrier 19 is turned in the same direction. As a result, the valve lifter 16 moves up and down via a boss 16e screwed into the threaded part 14a of the valve stem 14. Then, after a valve clearance between the cam carrier 19 fitted to the recess 16b of the valve lifter 16 and the cam face 20a of a cam shaft 20 is adjusted, the projection 19a of the cam carrier 19 is coupled to the notch 16c of the valve lifter 16.