Cassettes listed in the table below will fit most Shimano and Shimano-compatible freehub bodies, with the following exceptions: The ramps and shaped teeth of Shimano Hyperglide cassettes and similar cassettes from other manufacturers, listed in the tables below, improve shifting in the intended sprocket progressions.It is possible to mix and match sprockets with some small penalty in smoothness of shifting.You may also use older Uniglide sprockets, though these are becoming rare.
It is not difficult to customize Shimano cassettes.If you substitute an un-approved cog, Uniglide or Hyperglide, SRAM, IRD, Miche, or some other brand, it will still work, but the shift to/from that cog will probably not be as smooth as a Hyperglide shift normally is.If you remove the 21-tooth sprocket from a J, you can make it into a 13-19 corncob by buying an 18 to put between the 17 and the 19.Alternately, you could make it into a 12-18 by removing the 19 and the 21, and buying a 12 and an 18.As of 2010, many bicycles are still being sold with 7-speed freewheels. The trend toward more sprockets is driven largely by spec hype.
When you need to replace a wheel, you may upgrade to one with a 7-speed cassette Freehub and avoid having to change the shifters. Two chainwheels and 7 sprockets can provide everything most cyclists need. A third chainwheel adds much more than an 8th, 9th or 10th sprocket.
Since people managed without Hyperglide for several decades, this shouldn't scare you off.
In particular, if you substitute the top or bottom sprocket, you will only have one shift that isn't HG; shifts to or from the extreme sprockets tend to be less troublesome than intermediate shifts anyway.
The bolts or rivets only serve to keep sprockets and spacers in order when the cassette is not installed on the freehub.
They can be removed to replace sprockets or build up custom combinations.
For example, Shimano doesn't make any true "corncob" (one-tooth-jump) cassettes for time-trialists or flatland riders.