some stuff from sheldons site
Building Custom Cassettes
Shimano wants you to use one of its standard combinations, and offers a wide-enough choice to suit the needs of most cyclists, but you don’t have to if you don’t want to!
Shimano cassettes that don’t use spiders have most of the sprockets held together by 3 small bolts or rivets. These are not essential. Their function is convenience, in allowing the cassette to be installed slightly more easily. To make a custom cassette, you will often need to remove the screws or rivets. Just discard them: they are unnecessary in practice.
Some cassettes use screws with a 4 mm hex head. These can be removed with an adjustable wrench.
Some cassettes use screws with a 2 mm Allen head. These are frequently mistaken for rivets.
The easiest way to remove rivets is to grind off the heads where they protrude from the largest sprocket. I generally do this on a bench grinder.
It is not difficult to customize Shimano cassettes. If you substitute an un-approved cog, Uniglide or Hyperglide, it will still work, but the shift to/from that cog will probably not be as smooth as a Hyperglide shift normally is. 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.
For example, Shimano doesn’t make any true “corncob” (one-tooth-jump) cassettes for time-trialists or flatland riders. In 7 speed, the closest is the J (13/14/15/16/17/19/21).
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.
Similar modifications can be done with other ratios. Generally, the smallest sprocket needs to be one with a built-in spacer, designed for the top-gear position. While you can’t insert or remove a sprocket within a spider module, you can add sprockets on either side of these modules.
For example, if you want a 13-32 9-speed, you can start with a 12-27:
Replace the 12 and 13 with a top-position 13.
Add a spacer and a 32 after the 27
(you don’t need to buy the spacer, because you can use the spacer that came between the original flat 13 and the 14.)
There is no problem mixing 7-speed or 8-speed flat sprockets into a 9-speed cassette, or vice versa. The smaller sizes (11, 12, 13) that feature built-in spacers should ideally be matched, but even this isn’t generally a problem in practice. If you want to get finicky, you can compensate by using a wider spacer next to a narrower sprocket, or vice versa. On the other hand, if you use 9- or 10- speed sprockets with built-in spacers in a 7-or 8-speed cassette, the spacing will be too narrow for the wider chains used with these sprockets. You should use only 10-speed sprockets in a 10-speed cassette, because the internal width of the chain is narrower.
