Servicing the Derailleur Pulleys

Derailleur pulleys should be kept free of crud. Sometimes, a problem with the rear derailleur will be due to a bit of grass and mud wrapped around the base of a pulley. Clean the pulleys thoroughly, carefully checking the area between the cage and the pulley. A degreasing solution helps.

Derailleur pulleys can be damaged if the rear shifting is out of adjustment (the chain sits slightly off center, scraping the pulley), or due to mud- and sand-riding. Or, the bearings may freeze up from crud, rust, or wear.

Just looking at the pulleys is revealing!
Just looking at the pulleys is revealing!
Inspect the pulleys. If they don't spin freely, or if the teeth are gouged or worn down, replace them. Use pulleys specific to your derailleur: the top pulley for a grip shift (or friction shift) system usually has no side-to-side play, while an index-shifter (rapid-fire) may have 1/8-inch of side to side motion.
If you're not an old hand, considering drawing the path of the chain before taking off pulleys.
If you're not an old hand, considering drawing the path of the chain before taking off pulleys.
Note the position of the chain relative to the pulley you're replacing -- it's easy to get "lost." If the bike is upside-down with the wheel off, the top side of the chain goes around the most distant pulley, then over-top of the pulley closer to the frame.

Remove the pulley by unscrewing the hex-head bolt.

Sticking the new pulley in.
Sticking the new pulley in.
Slide the new pulley into position.

It doesn't necessarily have to be a store-bought pulley. You can take a pulley out an old broken derailleur, or mooch one from a friend.

Use thread-locker on any bolts that won't be adjusted routinely.
Use thread-locker on any bolts that won't be adjusted routinely.
Apply a thread-locking compound ("Loctite") to the bolt threads.

Now insert the bolt and tighten it into place. Done!