There comes a time in the life of every valve when it must either be replaced or reconditioned. Valve faces become worn and eroded as the seats wear, the stems wear where they contact the guides, and the tips of the stems can become worn or mushroomed from the constant friction with the rocker arms or cam followers.
Heat also takes a toll on the valves. The hotter the valves run, the more likely they are to undergo undesirable metallurgical changes. Exhaust valves run hundreds of degrees hotter than intake valves (1,200