Valve Adjustments and Cam Shafts
Most engine noises are the result of excessive clearances caused by wearing or worn components. Valve tappets wear and become convex, decreasing the amount of lift. If a tappet (they are replaceable) wears, and has been over shimmed, the valve will remain open too long; burning will result. The same is true for the pad or the adjustment screw for the rocker arm. Valve adjustment is important (vital) to engine performance. Be careful and patient when adjusting valves. There are many great tools out there to complete this task.
Valve Lift: Valve lift is the distance the valve is opened into the engine's cylinder. This is a result of valve train adjustments. In a camshaft in-the-block engine, this includes the camshafts lobe, the push rod, and the characteristics of the rocker arm ratio (1.5:1 or 1.6:1) etc. In an overhead cam engine, this involves the lift (distance from the center of the shaft) of the camshaft lobe and the tappet bucket.
Camshaft Duration: This is the amount of time the valve is open and off its seat. This measurement is taken in crankshaft degrees. This is the time the camshaft holds the valve open in a cylinder.