They wear, or in this case "seat", simply enough. When you adjust your valves you're setting the clearance between the rocker arm (or the tappits on an L head) and the valve stem. This allows the valves to shut all the way and effectively times them with every thing else. It's pretty common to have to readjust on a new/rebuilt engine after break in.why do you have to adjust them? the valve springs become loose or do the valves wear out easily. Is there anything you have to do other then take the valve cover off to check them? Can you tell by just looking under the valve cover or do you have to take the whole cyclinder head off.
Can you tell by driving it, if the valved need adjusting or replacement?[/b]