Modern Rear Derailleur Overhaul

Newer rear derailleurs are usually trouble free. Sometimes they get gummed up and might not spring back like they should. Or more commonly the pulley wheels stop turning freely. Here's a tutorial to take one apart and reassemble it. The derailleur shown is an older Shimano Light Action unit, but it's similar to other Shimano indexing derailleurs.

First you need to remove the tension pulley. Usually this is an 8mm hex or a 3mm allen bolt.

Now remove the guide pulley. I often just spin the cage plate counterclockwise to loosen the bolt rather than fuss with a wrench. Keep track of each pulley and its parts. The top pulley is different than the bottom one and shouldn't be switched.

Now clean the pulleys and the bushings well and reassemble them with oil. They should spin freely.

If the derailleur's springs are working fine you can just reattach the pulleys and stop here. Otherwise it's time to remove the remaining cage plate. This particular derailleur uses a 6mm allen bolt to hold the pulley cage knuckle together. Most derailleurs instead use a long slender 2.5mm allen bolt to keep the pulley cage in place. In either case, unscrew the bolt and the cage will pop right off.

 

 

With the cage plate removed you'll see a spring and a plastic dust shield. Clean these as well as the inside of the knuckle. Your derailleur may have 2 holes on the plate you removed which the spring attaches to. If your derailleur is worn and the chain tension is low, or you just want maximum chain tension to prevent it from falling off, use the stronger setting when you reattach the cage. If your tension is fine then use the weaker setting for less drag (this is usually the factory setting). 

Now you can also take the B knuckle apart. I really only suggest doing this if your derailleur is very old and dirty, or if it won't maintain an even chain gap between the top pulley and each gear. The backing plate can be very difficult to reattach since you have to fight the spring's tension. This doesn't apply to Sram derailleurs though, since there is no spring in the B knuckle to worry about. Anyway, first you have to pry out the C-clip:

 

 Now remove the spring and dust shield, and clean them. If the top bolt doesn't slide out or turn easily then it's probably bent, and this might cause an inconsistent chain gap between gears. If this is the case I would suggest clamping the bolt in a vise and bending it back with an allen key. If it doesn't come out but at least turns easily then don't worry about it and just squirt some oil in it. Here is the B knuckle taken apart:

Ok now it's time to put everything back together. First pack each knuckle with grease and reinsert the springs. The springs need to catch a small hole at the bottom of the knuckle. Rotate the spring until you feel it catch, then push it down into the hole. Try twisting the spring some more to make sure it's locked in place. After this you can put the dust shields back in place.

Now comes the hardest part. You need to rotate the B knuckle's backing plate against the spring's tension until the plate's tab is behind the derailleur's stop. Then you will need to push it down and hold it in place as you slide the C-clip back on. I can sometimes do this by hand, most of the time I need to wedge a small screwdriver between the backing plate's tab and the mounting bolt and twist until the tab is behind the stop. I like to use pliers to hold the backing plate in place once you twist it far enough. Once you get the C-clip in place then it's locked together and you can relax.

Now you can reattach the pulley cage. It's probably easier to reattach the pulleys and assemble the cage before attaching it to the rest of the derailleur, but I like making things harder for myself for no reason. Anyway with the spring and dust shield secure you can slide the end of the spring through one of the holes on the cage plate. Note that the spring has one end that's a smaller diameter than the other, the smaller end goes into the pulley cage. Next you will rotate the cage against the spring's tension until the plate's stop is behind the tab on the knuckle. This step is much easier than the last one since you can use the entire cage for leverage. Once the cage is rotated behind the stop you can push it all the way into the knuckle and secure the bolt:

If you didn't already assemble the pulley cage you can attach them now. First install the guide pulley (this one usually says "Centeron G-Pulley" on it):

Now rotate the second cage plate and add the tension pulley:

Make sure all the bolts are nice and tight, and then you're done!