WHAT YOU NEED TO KNOW
Replace a chain that has stretched: You need to replace a chain, if you have cycled so far that the chain has stretched beyond acceptable limits. A bike chain inevitably stretches with use, so you need to replace it before the now-too-long links wear down the teeth on your cassette and chainring, and you need to replace all three components.
Measure chain stretch: It is impossible to judge chain stretch with the naked eye - it just feels 'loose' or 'chewy' when you pedal. It is also impossible to say for how many miles a chain will last - it depends how clean you keep it, and how durable the chain. So use a Chain Wear Indicator to measure the length of your chain links.
Place one end of the longer side of indicator into the chain. This longer side might be marked '1%' wear, for example.
If the other end of the indicator drops into the chain, then the chain needs replacing now. The chain has stretched too much.
If the longer side of the indicator stays proud of the chain (see figure above), then the chain has not stretched too much, but how much exactly is not clear.
We need to measure some more with the shorter side.
Place one end of the shorter side of the indicator into the chain (perhaps marked '0.75% wear').
If the shorter side of the indicator will drop into the chain, then the chain needs replacing soon (see figure above). The chain has stretched a little, but not too much
If the shorter side stays proud of the chain, then the chain has not stretched much at all - it is fine.
Check your cassette and chainrings:
If the answer to these questions is "Yes", then, unfortunately, you probably do need to replace all three components at the same time (see Replace cassette page and Replace chainring' page). You did not replace your chain soon enough and are now being punished!
Check your chain: Before you buy expensive bits, check your chain is running freely. Kinks in the chain, or rust/severe lack of lubrication may also cause your chain to 'kick off' (see Check chain runs freely section on this page)
Split chain: Place the chain into the central channel of a chain tool or 'Chain Splitter'. Align the pin of the chain tool with the rivet of a link of the chain. Turn the crank handle of the tool so that the tool slowly pushes out the rivet. Then turn back the handle, retract the tool pin, and the chain will be split.
If you will be putting the same chain back on, the trick is not to push the rivet out too far - just enough to clear the barrel and for the chain to fall apart, but the rivet is still stuck in the outer link plate. If left like this, the rivet will be a lot easier to push back in.
Keep the old chain: it's a good guide to the correct length of the new chain.
Thread one end of the new chain through the rear Derailleur mechanism - over the front of the upper jockey wheel, through the hanger and then round the back of the lower jockey wheel. Then thread the other end through the front mechanism. Keep the chain slack by threading over the smallest sprocket of the rear cassette and the smallest chainring.
Check chain length:
Decide if you need to shorten your chain:
identify the link(s) to remove. Check that the chain ends you will be left with are compatible e.g. if your new chain has a connector link then you'll want two, closed ends.
Then, use the chain tool as before to push out the rivets. Take care not to shorten your chain too much - is a lot easier to take links out, than add links in.
Re-usable links or chain connectors come in two halves. Put a half-link on one end of the chain, and the other half-link on the other end.
Then join the two halves of the link together, and pull the chain ends apart. Hear it click and it's joined!
Turn the chain tool round so that the chain tool pin is on the same side as the rivet sticking out of the chain. Then place the two ends of the chain in the tool, with the barrel end of the chain between the open link plates of the other end of the chain.
Align the pin head of the tool with the rivet, the hole in the outer link plate and the hole through the barrel. Mmm ... fiddly! Then screw in the pin to close the chain. If the chain is not lubricated already, oil it now.
Test your new chain: Pedal gently, and attend to the chain closely.
Can you hear any clicks or see any jumps as the chain passes through the Derailleur?
Can you see any 'kinks' or 'humps' in the chain as its moves (see figure)? The chain may even 'kick-off' the gears, particularly under pressure.
Pushing the rivet back in may also squeeze the plates together, creating kinks in your chain.
Identify which rivet is causing the problem, and then widen the link
Try inserting a screwdriver into the link and then rotating the blade, forcing the link plates fractionally away from the barrel.
Or place your thumbs on the sticking link and attempt to bend the chain laterally (sideways, rather than up and down).