It is probably a combination of the above answers, plus heat. I would think heat is the primary cause of the stretch. As you know metal expands, when it gets hot. It will also shrink when cooled. There is probably a point where they will only shrink back to a certain spot and still remain in the stretched position.
Gets warped by the heat and friction over time
Valves do not stretch rods do. Valves just push up in to the seat as they are used.
All metal bend and even wear down. Valves don't stretch. They wear down. Good Luck...
Valves do not stretch - where are you getting these ideas! If you're wondering how valve clearances get tighter - it's from the valve face pounding into the valve seat. The seat gets worn down and the valve gets worn down, causing the valve to get pulled higher into the head.
