Within a given brand, 7-speed, 8-speed, and even "ultra-6"-speed cog-to-cog spacing should be close enough to permit indexing to work properly, provided that you have enough range in your derailleur and shift levers.
Converting from 7 speeds to 8 requires a 4mm longer axle and a wider plateau on the freehub body. Sheldon has a useful article on Shimano's various systems and how to interchange among them.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069
Last edited by John E; 02-12-02 at 08:54 AM.