K I need some help with this because I've gotten a couple different answers now. No guessing. Is the only difference between an 8 speed cassette and a 9 speed cassette in the spacers or are the cogs actually thinner and thicker or teeth different? What I'm wondering is could I build a cassette with 8 speed cogs, but use 9 speed spacers so that it would work and shift normal and all that? I finally found a place that will sell me just a 21 tooth cog for like 5 bux but only in 8 speed, and I won't run an 8 speed chain. Don't ask. Anyone know?