Once your mountain bike chain becomes damaged, you should immediately replace it with a new one.
It is possible however, to repair a broken chain using a chain tool. For this very reason, most mountain bikers travel with a chain tool.
Your chain has three basic components – the metal side plates, the rollers between the side plates, and the rivets, or pins, which go through the rollers and help to hold the plates together. These pins allow the rollers to freely turn as the chain moves around the cogs.
What to do if your chain breaks?
If your chain happens to break, you’ll need to remove the broken link and replace it with a spare link. To do this, simply reattach the two ends of the broken chain and ride on a shorter chain until you can get it replaced.
To remove a broken link of chain, place it in the chain tool. Now, turn the tool counter clockwise until the rivet pin of the chain tool touches the chain rivet. Continue to turn the tool until the pin pushes out of the roller. Be very careful, as you want to stop turning when the pin is right at the edge of the roller, before it moves through the outer side plate.
Now, turn the tool in the other direction, and back it out of the roller. Set the tool to the side, then work the chain very gently from side to side and extract the inner side plates and roller.
Now is the time to re-route the chain through the bike. You may want to have a chain retaining tool or some to help you hold the chain in the right spot as you route and repair it.
Now that the broken link has been removed and you’ve re-routed the chain, you’re ready to insert a new link or simply connect the links that were beside the broken one. The process here is the same – align the two ends, so that the link with the inner side plates will fit inside the link with the pin and outer side plates. Now, use the chain tool to push the pin inward until it’s positioned evenly between the side plates.
The easiest way to learn how to do this or feel comfortable doing it is to have someone show you, then actually practice with a chain and a chain tool. You’ll have no trouble at all making a temporary repair in a mountain bike chain once you’ve seen it done by a professional and practiced it yourself a few times.