The suface of the cam followers in contact with the cam shaft are 'case hardened'. As the engine gets older this hard casing breaks up and the followers become badly 'pitted' the casing that is left around each pit mark thus leaves sharp hard edges to the pits (a bit like a pot hole in the road). These pittings then serve only to distroy the cam lobe by shaving the metal off them. Once it has started there is no way to stop it. Prevention is the only way to keep your engine in good condition. A 'collapsed' hydraulic tappit should be investigated and rectified immediately, there should be no tappit noise, since the hammer action of the cam against the follower can chip the case hardening off the follower. The cam rotates at half engine speed, so any 'tapping' at that speed may indicate a collapsed tappit. Making sure you have the correct grade of oil for the operating temperature and allowing your engine to warm up before any 'speed' demands are put upon it, is very important. Thick oil in winter is a killer! once the engine has been run in, an oil additive has proved to be helpful (we used STP or Wynns), this these products leave a coating of lubricant on the mating surfaces of the follower and the cam lobe so that (especially when left for a while) the components do not start up in an unlubricated 'dry' state.
Rgs. Bob.