Age is not kind to metal; whether it just sits or is used continuously. Mosture cycles through hot and cold phases between shafts and bores. I have pulled rocker shafts from heads where some of the aluminum of the head came out with them: heads are then useless; replacement necessary.
In an effort to save space on the secure site, I'll continue this on the information site: Details. I've run out of space in the main area; you'll have to register and log in - details are not checked or used; just make something up.