Stretched chains ruin sprockets - because they are out of pitch, they put all of the load on one sprocket tooth. You can usually see the wear by comparing how pointed the tips of the sprocket teeth are, alongside a new one. If your sprocket's teeth are very pointed, or have a long taper instead of widening rapidly from the tips, they are done for. Really terminal wear results in the tips of the teeth being undercut so far that they break off. Putting a new chain on a worn-out sprocket tends to ruin the new chain fairly quickly - it puts all of the load on one link instead of spreading it around the sprocket. If the sprockets are worn, it's much better to replace them simultaneously with the chain.
Meanwhile I hope you followed Darryl's instructions on tensioning your chain. The old front one looks rusty - keep your chains properly oiled if you want them to last. I like chainsaw bar oil for the job. Rusty chains have a very high wear rate (and then they ruin your sprockets too).
Last edited by grumpy; 03/06/09 07:00 AM.
