Shimano has been producing and making bicycle components, including shifters, for several decades. The company is known among the cycling community for the high-quality products and components coming from its factories. Look at most bicycle races or mountain bike events and you are sure to see a majority of the bikes equipped with these components. If your Shimano shifters are starting to get a little sloppy in the gear-switching, you can fix them in your own workshop or at home.
Place the bicycle into the bike work stand and secure the bike stand collar. With one hand turn the bicycle pedals and with the other run through the gears by using the Shimano shifter. Listen for metallic grinding noises and watch at the cassette for missed gears or sloppy shifts.
Turn the barrel adjustment --- a small, round piece of black plastic on the inside edge of the shifter --- a quarter turn to the right and try running through the gears again. Check whether this tightened the shifting process. Keep adjusting until the shifts are smooth and no gear is missed on the cassette. If this does not fix the problem, proceed to Step 3.
Locate the Allen screws attaching the shifter pod housing together. These are under the shifter pod housing. Turn them to the left with the Allen key until you can remove the top piece of the shifter housing. Inspect the inside of the shifter for grit and debris. Look at the cable and check whether it is worn or without grease. If there is no grease, add a dollop to the exposed cable wire.
Turn the bearing cartridge one half-turn to the left and remove. The bearing cartridge is a round metal circle packed with a series of small stainless steel ball bearings. There should be a healthy quantity of grease in the bearing cartridge. If there is not, replace with the new cartridge. Set it into the housing and turn it one half-turn to the right. Replace the housing top and tighten the set screws with the Allen key by turning them to the right until firm and tight. Do not overtighten because this may crack the plastic housing. Run through the gears and check to see that no gears are missed on the cassette. If this does not solve the problem, proceed to Step 5.
Remove the top of the shifter and use the Phillips screwdriver to loosen the cable-set screw. Turn the screw to the left until you can remove the cable. Pull the cable off the frame cable housings and go to the derailleur it is attached to. Use the Phillips screwdriver to loosen the set screw on the derailleur by turning to the left until you can remove the cable.
Thread one end of the new cable into the set screw on the derailleur and tighten the set screw by turning it to the right until the screw is tight and the cable is locked into position. Thread the cable through the cable housings along the frame and thread the end into the set screw in the shifter. Tighten the set screw by turning to the right until firm. Place the shifter housing back into position and tighten into place with the Allen key.
Run through the gears, checking for slop in the shifter and off the cassette. Turn the barrel adjuster until the shifting is smooth in both the shifter and cassette.