Bicycles/Maintenance and Repair/Chains/Mending a broken chain

Mending a broken chain
Newer, narrower chains and wider chains for single speed or three speed bicycles often have special links for removing the chain, but the chain tool is still needed to remove excess links when replacing chains.

A chain tool can be used to push out the rivet which joins the plates of the chain together. Many chain tools have two positions the chain will fit- make sure the plate furthest from the extractor pin is supported by the tool. Once this is done the chain can be replaced or re-assembled as a shorter chain. Most chains will have enough slack to allow removal of a few links without making the chain too short.

Avoid pushing the rivet all the way out, it should remain lodged in one of the outer plates. A small portion of the rivet left protruding on the inside of the plate can hold the chain together during reinstallation. Use the same position in the chain tool, with the chain plate furthest from the chain tool's pin supported, and push the rivet back into place. After reinstallation, the link will usually be too stiff- leading to chain skip unless loosened. The other position in two-position chain tools, with the closer plate supported by the tool, is used for loosening tight links. Force the side of the rivet that protrudes more into the link to loosen the link. Alternatively, grasping the chain on both sides of the tight link and flexing the chain in and out will loosen the link.

Note
Although they're made of metal, chains seem to "stretch" as the links wear. This will cause them to not engage on the gears properly (they will hook on to the middle or top of the tooth, instead of the bottom of the groove). This will also cause the gear teeth to wear down.

To test for excessive wear on a chain, open a link and remove the chain. Then try to flex the chain sideways, in the direction it is not supposed to bend. If you can make anything more than a 1/8 arc (for example, if you can make a half circle or if you can touch the ends), then your chain is worn and should be replaced. Alternatively, measure a straight section of chain under slight tension. Standard links of chain measure one inch when new- if 11 links of chain measure 11 1/8" (283 mm) or more the chain should be replaced.

If you only replace the chain and not the gears, the chain may skip. For this reason, it is best to replace both at the same time (the front gears are not as affected by this because they are bigger and the chain doesn't pass over each tooth as many times). Replacing the chain more often, when 11 links measure 11 1/16" (281 mm), will allow the gears to be reused.

Avoiding Chain Wear
To prevent such wear, avoid using gear combinations that stretch the chain diagonally.

Also change gears only when you are turning the pedals. Remember to gear down when coming to a stop.