Shhhhh! don't use the word "stretched" in the same sentence with chain. Some people here take the word stretched too literally. And they get very confrontational about how a person can't stretch a chain.
Wear is a much safer term around here! <grin>
For 11 speed chains, I've started replacing them when the pin closest to the 12" mark gets to be about 1/16" further. For the old bikes with 7 speeds and less, I never worried much about either chain or cogs, just replaced when one didn't work with the other.
Chains are about the only thing with the drive train that I do preventative maintenance on. I'm happy to let the rings and cogs go until they show me issues. Which is usually the chain skipping or slipping on them when putting some power into the pedals to accelerate rapidly from a stop.
Last edited by Iride01; 08-07-22 at 11:44 AM.