One of the most aggravating problems you can have with a bike is if the gears don’t change properly. So in this article, we are going to show you how to remove, replace and adjust a Shimano rear mech.
For the job, you are going to need a set of Allen keys, a Philips screwdriver, some pliers, a chain tool, and possibly some cable cutters.
Removing your chain
You’ll first need to remove your chain. If you have not already done this, you can find out exactly how to do it with our article here. To remove a Shimano rear mech, you will need to detach the cable. Put the mech to the smallest sprocket using your STIs (Shimano Total Integration), pull the cable crimp off the end of the inner cable using some pliers. Then undo the cable crimper bolt by unscrewing it anti-clockwise. With a Dura-Ace model this requires a four-millimeter Allen key, but for some old models, you may require a five-millimeter. Once you have done this, you will be able to pull the cable free from the mech. Finally, simply unscrew the main five-millimeter rear mech bolt and the derailleur will come free from the hanger.
Attaching the rear mech
To attach the rear mech back to the frame place, the main bolt against the rear mech hanger and use your five-millimeter Allen key to turn it clockwise until it bites. Once started, it should be easy to tighten, if it isn’t it might be cross-threaded so remove it and start again. Before you tighten it too much, make sure the B-tension screw on the rear mech is behind the stop from the hanger. This is what keeps the rear mech spring loaded. This B-tension screw is used to space the top jockey wheel away from the largest sprocket. If the jockey wheel’s too close, then the chain won’t shift into the top sprocket because they will be overlapping. Unscrew it anti-clockwise until there’s about a one-centimeter gap between the top jockey wheel and the largest sprocket.
Rusty, frayed, or old dirty cables will prevent accurate shifting.
So if you are in any doubt, it might be a good idea to replace them at this point. To take the inner cable out, first , you make sure the STI’s been clicked right the way down to the lowest gear, then roll the rubber part of your STI leave it back to expose the end of the cable. Then find an area where the inner cable is exposed and push it through using your hands. The end of the cable will start protruding out to the STI leave and you will simply be able to pull it through from there. To thread a new cable in, push the end through the hole on the outer side of the STI lever until it appears on the inner side, and then just pull it through until the cable end locks into the lever. Thread the cable through the first part of the outer cable near the bars then through the cable guide located underneath the bottom bracket. And finally through the last piece of outer cable and through the barrel adjuster of the mech. Place the ends of the outer cable into the cable stopping the frame and the barrel adjuster. The barrel adjuster should be screwed almost right the way in. we tend to screw it right the way in and then unscrew it one turn just to allow for some adjustment either way. Pull the cable through the crimp bolt on the mech, which side you put it on depends on the model, but you will be able to see a groove where the cable should sit, pull it as tight as you can by hand, and then tighten the bolt clockwise. Next, you will need to reattach the chain. You will then need to adjust the high and low stop screws. These are not for indexing your gears, but simply limit the maximum travel either direction to the mech. Make sure the STI leave is still in the lowest position for the small cover on the cassette, and that the chain is on the big ring on the front. Then adjust the high screw marked with and H, screwing it clockwise will move the mech towards the bigger cogs, while unscrewing it will move the mech in the other direction. You want to adjust this screw so that looking from the rear of the bike the top jockey wheel is directly below the smallest cog on the cassette. If the mech is not low enough, the chain won’t shift onto the smallest cog. And if it’s too low, the chain might jump off the cassette and get jammed against the frame, potentially causing a lot of damage. Once you have done this, put the front mech down to the small ring. And move the rear mech up to the biggest cog using the STI levers. If the chain won’t go all the way up to the biggest sprocket, it means either there’s not enough tension on the cable or the L-screw is too far in. To check if it’s the cable tension push the mech up using your finger and thumb while turning the pedals. If it then goes into the big sprocket then it’s the cable tension that is the problem. However, if it still won’t go into the big cog when you push the mech, then you need to loosen the L-screw. Turn the pedals at the same time and loosening it. At this point, the chain should jump up onto the biggest sprocket. Have a quick from behind to make sure the upper jockey wheel is in line with the biggest sprocket. Now, the other extreme, if the L-screw isn’t far enough in the chain, you can jump over the biggest sprocket and get jammed between the cassette and spokes. If however, you can push the mech by hand onto the top sprocket, then you need to increase the cable tension to do this unscrew the barrel adjuster on the rear mech anti-clockwise like this until it jumps onto the biggest sprocket.