The cam lobes are ground on a one degree angle and the bottom of a new lifter is slightly rounded which is what causes the lifter to spin as the engine is running. If the lifters don't spin the cam lobe, which is not hard, will wear away as well as the lifter. When you first fire the engine up and run it for awhile the two wear in together and generally wear a long time depending on valve spring pressure.In my book it isn't worth the risk of a new engine to use an old cam and lifters because if it goes away the metal particles will contaminate the whole engine and you get to start over. Good luck.
