Chains stretch over time and use. Oil them well and often and they last longer.
Measure your chain with an old style American ruler (with 'inches'). Twenty links should be exactly 10 'inches'. [0] If it's 1/8" 'inch' longer, replace the chain. If it's 3/16 'inch' longer, replace the rear cogs and the chain both. Consider replacing the front chain rings, too, especially if the chain is longer or they're made of aluminium.
A stretched chain ruins gears. Ruined gears will ruin a new chain, too. Proper maintenance fixes both problems. New quality chains cost about US$10-15.
[0] Wikipedia says one 'inch' is 2.54 centimeters, if you believe that.
Measure your chain with an old style American ruler (with 'inches'). Twenty links should be exactly 10 'inches'. [0] If it's 1/8" 'inch' longer, replace the chain. If it's 3/16 'inch' longer, replace the rear cogs and the chain both. Consider replacing the front chain rings, too, especially if the chain is longer or they're made of aluminium.
A stretched chain ruins gears. Ruined gears will ruin a new chain, too. Proper maintenance fixes both problems. New quality chains cost about US$10-15.
[0] Wikipedia says one 'inch' is 2.54 centimeters, if you believe that.