Stefan Mileschin

A Rohloff chain wear gauge is just one of the dozens of similar tools available. Ian slack

Using a chain checker to measure how much your bicycle chain has stretched will save you big bucks in maintenance costs over the lifespan of your bikes. Just don’t forget to use it!
Wait. What? Bike Chains Stretch?

It seems counterintuitive that anything so heavy and made of steel could stretch, but bicycle chains do. It’s not that the steel itself stretches. What happens is all the little rollers, bushings, and pins that make up a chain wear, and as that happens the chain’s “pitch”—the distance between each link—grows ever so slightly. The industry standard pitch for derailleur chains is a half-inch (12.7mm) spacing between the pins. A chain is considered significantly worn when it exceeds that standard pitch by one percent. Because the chain has to fit into the teeth of the sprockets and chainrings, what happens is a stretched chain puts more pressure on the sides of the teeth, causing them to wear faster.
If you put a ruler next to the links, you can see how the pins line up every half inch on a standard pitch chain. Ian Slack
Stretched Bike Chain Symptoms

In time, a chain and the rear cog grow to “mate” with each other, and if you try to put a new chain on a worn cog, it won’t fit into the worn teeth properly. It will actually skip as the chain slides over the teeth. It’s usually most noticeable under pressure going up a hill and can be very unpleasant if you’re standing up on the pedals when the crank suddenly lurches forward and threatens to send you over the handlebars.

You can ignore the problem and let a chain and cog grow old together. But what happens then is you eventually get chain “slop” which is extra side-to-side flexibility as the chain wears. A sloppy chain doesn’t respond to the action of the derailleurs as well, and you get poor shifting and other problems like additional noise.

https://www.reviewgeek.com/23870/how...e-maintenance/