To remove a “current generation” of Shimano’s Hyperglide cassette from the freehub of a rear wheel you will need two rather specialized tools besides a 12″ adjustable wrench. First, the lock-ring that holds the cassette on the freehub has 12 internal splines, as can be seen in the first picture.
Park Tool’s #FR-1 or #FR-5G will fit this lock-ring. FR-5G has an alignment pin that makes the job of loosening the lock-ring a lot easier. Without the pin, the tool is hard to hold in place while also holding the cassette with a chain-whip. Using a quick release skewer lightly snugged up will also hold the tool in place. In the second picture you can see FR-1 on the left and FR-5G on the right. The third picture shows FR-5G in place for loosening the lock-ring.
The next tool you will need is some kind of device to hold the cassette from rotating backwards (counter-clockwise) as you loosen the lock-ring. Large ChannelLock-type pliers will not do it as they tend to bend or nick the teeth of the cogs of the cassette. An old piece of chain used with a pair of Vice-Grips will hold the cassette well enough for loosening. The best way to hold the cassette, however, is to use Park Tool’s SR-1 chain-whip, or equivalent, positioned as seen in picture four, to the left.
A 12″ adjustable wrench, a 1″ open-end or box wrench, or even a 26mm open-end or box wrench will turn the FR-5G tool to loosen the lock-ring, as seen in the fourth picture.
Holding the chain-whip, turn the adjustable wrench counter-clockwise to loosen the lock-ring. The lock-ring and the smallest cog of the cassette have serrations on their mating faces to help prevent loosening while riding, so you will hear some clicking as you loosen the ring. Once it is loose, remove the chain-whip and wrench and also the ring. It should look like picture five.
Now, simply lift the cassette off of the freehub, but be careful to hold onto the two smallest cogs of the cassette because they will not be firmly attached as the rest may be. Take special care to notice how they fit onto the cassette for replacement later. Notice how their flanges face, and how they also have a larger spline tooth and smaller groove like the rest of the cogs on the cassette. These must all line up.
Most cassettes are held together by a single screw that keeps all but the two smallest cogs together for easier handling and assembly. Some cassettes have all but those two smallest cogs rivetted to an aluminum spider, too! But, some cassettes have all of their cogs and spacers simply assembled onto the freehub’s splines one-at-a-time. Be very careful about keeping everything in order and be sure you know exactly how all of the spacers and cogs fit onto the freehub. Each spacer has two small pins that MUST fit into the correct holes of the cogs. If they don’t, they will hold two cogs just slightly too far apart and that will ruin the alignment that allows for index shifting. If you are not absolutely sure you know how all of the spacers and cogs fit together, then if they all feel loose as you try to lift the cassette off the freehub, stop and take it all to your local bike shop for servicing.
The freehub with the cassette removed should now look like picture six.
The majority of freehubs are made from hardened steel. Some road freehubs were made with aluminum spline shells to help save weight. Once the cassette is finally removed, these softer splines will usually show where the cogs have dug into the edge of the spline over time and under heavy loading. It will also make it hard to remove the cassette on these freehubs.
To replace the cassette on the freehub, just line up the large spline tooth of the cogs with the large spline groove in the hub’s shell and slide the cassette onto the hub. Be careful to get the flange of each of the last two cogs facing the wheel. Screw the lockring on and tighten it with the special tool and wrench. It doesn’t need to be “stand-on-it” tight. Park Tools lists it as needing about 260-434 in/lb. (about 21.7-36.2 ft/lb). Since the freehub’s ratchet will stop the cassette from turning clockwise as you tighten the lock-ring, you do not need the chain-whip for installation of the cassette.