A rear derailleur is a mechanism that attaches to the rear of the bicycle frame and allows you to shift the chain onto a set of gears known as the cassette. Before upgrading a Shimano SIS rear derailleur, make sure the replacement is compatible with the current drive train. This information can commonly be found in the product description, or by checking with a knowledgeable mechanic at your local bike shop.
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Things you need
- 5mm Allen key
- 3mm Allen key
- All-purpose bicycle grease
- Derailleur cable end cap
- Needle nose pliers
Use the rear gear shifter to shift the chain onto the smallest rear cog.
Locate the derailleur cable pinch bolt on the underside of the Shimano SIS rear derailleur. Loosen the pinch bolt with a 5mm Allen key.
Use needle nose pliers to pull the protective cable end off the tip of the cable. Pull the cable free of the derailleur.
Locate the lower of the two rear derailleur pulleys. This pulley, called the idler pulley, has a 3mm Allen bolt at its centre.
Use a 3mm Allen key to loosen and remove the bolt. Remove the pulley from the derailleur, enabling you to extract the chain from the derailleur.
Use a 5mm Allen to loosen the pivot bolt at the centre of the Shimano SIS derailleur. Pull the loosened derailleur away from the bicycle.
Separate the derailleur from the chain.
Use a 3mm Allen key to remove the bolt from the replacement derailleur idler pulley. Remove the pulley from the derailleur.
Grease the pivot bolt threads on the backside of the replacement derailleur. All-purpose bicycle grease works fine.
Attach the derailleur by way of the pivot bolt to its designated hole in the bicycle frame. Use the 5mm Allen key to fully tighten the bolt. Be sure the chain passes over the upper pulley.
Engage the idler pulley against the inside of the chain, and push the pulley and chain into place inside the derailleur cage. This is the same position the pulley was located before removal.
Thread the idler pulley bolt through the centre of the pulley, and tighten the bolt with the 3mm Allen key.
Pass the derailleur cable back beneath the replacement derailleur so that it passes between the derailleur and the pinch bolt.
Locate the small barrel-shaped nut at the rear of the derailleur. Turn the barrel all the way into the body of the derailleur, and then give it two full turns in the opposite direction.
Grasp the tip of the rear derailleur cable, and pull the cable tight. Use the 5mm Allen key to tighten the derailleur cable pinch bolt onto the cable.
Fit a new protective end cap onto the tip of the cable. Use a pair of needle nose pliers to mash it in place.
Use the rear gear shifter to shift the derailleur and chain to the second smallest rear cog. If the chain refuses to shift or hesitates before shifting, turn the barrel-shaped nut in toward the bike and try shifting again. Continue this adjustment as needed, or until the derailleur and chain offer crisp response.
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