Valve springs do get tirred after years of use. The radical temperature changes are enough to make any part tired. It is safe to say that strong springs are a must to put up with the rigors of everyday running, not to menton the stresses put on a high-performance valve train. New stock springs are a must for the average engine rebuild. When you graduate to the ranks of high-performance, you must upgrade to heavy-duty or dual springs to keep valve float at bay.
Valve Spring, All Models All Years, Stock Single Spring, Sold as Set