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A bicycle chain is the central component of the drivetrain, so it's important to replace it when it gets worn. There's no set mileage limit on bike chains, but there are several common indicators that it's time to replace a bike chain, the most important being stretch. Environmental factors also play a part in how often a cyclist needs to replace a bike chain. If the bike chain is particularly worn or you've has been riding the bike for a long time without maintaining the chain, it might be necessary to replace other components of the bike too.
There's a lot of dispute among cyclists as to how long a chain can last, but most agree that a well-maintained chain can last for at least 1,000 miles (about 1,609 km). The environment that bike is ridden in plays a large part in how long a chain can last too. Chains ridden in gritty or wet environments tend to wear out faster than others, though proper maintenance can make them last longer. You can usually get a good idea of how often you'll need to replace the chain on a bike by keeping a record of your mileage between replacements.
One of the main warning signs that it's time to replace a bike chain is that the chain starts to slip when you put pressure on the pedals. Other indicators that you might need to replace a bike chain may be sloppy or slow shifting; mis-shifts, in which the chain does not move into the gear you want it to shift to; or dropping the chain — which occurs when the chain skips off the front chainrings and lands either on the frame or off the far side of the crankset.
If your bike chain starts showing signs of wear, you should check the chain to see if it's stretched. This happens when the pins holding the links of the chain get worn down, which allows the links to stretch out further than they're supposed to. You can check this with a chain gauge or just with a ruler. To do this with a ruler, you should hold a ruler above the chain with the zero mark at the middle of one of the pins. Then look 12 pins down — on an imperial units ruler, this should line up with an inch mark. If the center of the pin is more than 1/16th of an inch (about 1.58 mm) past the mark, you should consider replacing the chain; and if it's more than 1/8th of an inch (about 3.17 mm), you should certainly replace it.
If you've been riding the bike for a long time without checking the bike chain, or if the chain is particularly worn, you should also check on the other parts of the drivetrain to make sure they're still OK. If you put a new bike chain on a very worn drivetrain, you could still run into shifting problems. Since the gears and chain are supposed to work in unison, if you put a new chain on a worn down cassette or freewheel, then the new chain won't line up properly with the old cogs. Additionally, old cogs can wear out a new chain more quickly than it should. If you replace your chain regularly, however, the cassette and chainrings or freewheel should last for much longer than the average chain.