Cassettes and chainrings keep getting more and more expensive, so it's getting more and more important to keep tabs on your chain stretch. There are a number of neat chain-checking devices out there, but chances are you've already got one in your junk drawer, even if it's not all that neat. A simple imperial ruler can tell you what you need to know. From pin to pin, every bicycle chain is exactly a half inch when it's brand new. Once it stretches 1 percent beyond that, it's gone too far. That's hard to tell in just a half inch, but it's a little more manageable over 12 inches. Line up one pin at the center of the "zero" on your ruler, and one foot later, if it's stretched more than one sixteenth of an inch, it's time to replace your chain. See? That's one thing the metric system isn't ideal for.