One of the most crucial components on your bike, a chain converts the force you apply to your pedals into forward propulsion on the rear wheel. Installing a new chain on a 21-speed bike is no more complex than putting a chain on any other bike. According to bicycle mechanic Alex Ramon, the most common type of chains do not have a quick-release link, but you can still remove and install your chain in a reasonable amount of time.
Rotate the handle on the chain tool counterclockwise until the push rod is fully backed out of the link rests. Place one of the chain links on the link rest that is farthest from the push rod on the chain tool. Turn the chain-tool handle clockwise until it pushes the pin out the other side of the chain link. Continue tightening the handle until the pin is held into the chain by just the outermost chain plate.
Loosen the chain-tool handle until the push rod is withdrawn from the chain and you can take it off the tool. Twist the link you loosened slightly and pull until the old chain comes apart.
Feed one end of the replacement chain over the top of the lowest pulley in the derailleur. Reach under the derailleur where the leading end of the chain is hanging and wrap it around the high pulley in the derailleur. Continue feeding the chain through the mechanism until you have enough links to wrap over the smallest gear sprocket on the rear wheel hub.
Pull the chain through the derailleur and wrap it over the largest chainring on the front of the bike. Bring the two open ends of chain together somewhere beyond the pedal cranks so you can work freely on them. Force the open chain links together so the chain plate of one hooks over the chain pin of the other link.
Place the chain tool on the chain so the chain pin is pointing toward the push rod of the chain tool. Turn the chain-tool handle clockwise until the chain pin is fully inserted into the link. If the links are stiff, set the chain on the link rest closest to the push rod and tighten the handle a quarter-turn at a time until the pin snaps into place and the links move freely.
Turn the pedal crank by hand to ensure the chain rides smoothly on the gear sprockets. Apply a drop or two of chain oil every few links as you turn it to lubricate the chain. Once the chain is fully lubricated, your 21-speed bike is ready for use.