A rocker arm is mounted to an eccentric shaft. The shaft has a pair of ends which are mounted to corresponding supports. The shaft has a rocker arm seat portion which is eccentric to the end portions, whereby when the position of the shaft is changed by rotating it, the position of the rocker arm changes. The shaft ends are mounted to riser blocks which are connected to a support structure. The shaft ends may be clamped in a fixed position relative to the riser blocks, or unclamped to permit the shaft to be rotated. When rotated, the position of the rocker arm may be changed up or down relative to the support structure and a valve stem which the rocker arm is intended to engage, thereby adjusting valve clearance.