Scrub the rocker shaft and with it roll it on plate glass; rarely it may be bent and if present it is advisable to acquire new ones. Make sure that the surface of the shaft is smooth each of the rocker arms does not leave any worn ring ,and replace the shafts if they are worn. Check the rocker arms for wear on the rocker bushes, on the face that rock against the stem of the valve and of the faces which ride on the Rocker shaft. To determine whether the rocker arm bush is worn, the rocker arm tip should be held and placed on the shaft and rocked while being observed for lateral shake. In case, both shake is present and the arm is loose on the shaft, then a new bush or the rocker arm should be fitted. Also it is required to check the condition of the roller connecting with the end of the valve stem and the slide pads coming into contact with the cam lobes and replace them if worn out.