Originally Posted by
FastJake
7-speed and 8-speed hyperglide are spaced the same. You can run an 8-speed cassette with 7-speed shifters (or vice versa) no problem. 9-speed and 10-speed are different though.
They are close but not exactly the same. Center-to-center spacing for Shimano 7-speed cassettes is 5.0 mm and for 8-speed cassettes it's 4.8 mm. Over 7 cogs the mis-match is 1.4 mm which can cause rough shifting and noisy running at one end or the other.
You can usuallu make it work adequately by getting the rear derailleur centered exactly over the 4th or 5th cog on an 8-speed cassette so the misalignment is minimized at both extremes.