Quote Originally Posted by Grog
My chain has been overextended for a while but it hasn't impaired the capacity of the bike to shift or anything yet. My mechanics wants to wait until it becomes a problem to change it. It helps that he sells me parts at cost. But now I'm starting to think that I might even have to change the cranks when that day comes and even at cost it's going to be expensive (Ultegra here).
When the chain gets stretched excessively (probably considerably greater than 1%) it doesn't fit the chainrings properly and you'll see a gap between the chain and the bottom of the teeth. This will wear the teeth into needle sharp points, and the chainrings (not the whole crank) will need to be replaced. I did this to my chainrings once. Not sure if it really happened because of chain wear of just a softish alloy in the Stronglight chainrings with lots of miles on it. And I've seen one other bike with a chain stretched to this extent. But I think it's pretty rare. I'd monitor the shape of your chainring teeth and whether the chain sits solidly at the bottom of the teeth. If you get worried, well, chains are much cheaper than chainrings.