An OHV engine comprises a first and second cam carried by a camshaft rotatably supported by a crankcase to be rotatively actuated by a crankshaft, first and second lower rocker arms pivotally supported by a lower rocker shaft adjacent to each other, and configured to be actuated by the first and second cams, respectively, a clutch member mounted on the second rocker arm in an axially slidable and rotationally fast manner, the clutch member being axially moveable between an engaged position and a disengaged position, the clutch member being provided with an engagement recess configured to receive a corresponding engagement projection of the first lower rocker arm for a joint pivotal movement of the first and second lower rocker arms when the clutch member is at the engaged position, and an actuator for causing the clutch member to move axially between the engaged position and the disengaged position.