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I have just bought a second hand mountain bike for my sons and have noticed that the rear derailleur is a sunrace M30 seven speed. The issue is that the bike is a 8 speed (8 cogs on the back)does it matter or will it work the same?
Does it work?
Remember that the derailleur doesn't know how many sprockets there are, at least not until they get 10 speed or so...
8-speed cassette needs an 8 or 9-sp chain and an 8-sp shifter.
Most Shimano rear mechs up to and including 9-speed will work for 6,7,8 or 9 but not 10-speed.
If the rear mech moves far enough in and out then all is fine.
I have had mechs that did not move in or out (different mechs) to cover the range required on mix-n-match hubs and frames.
Final point, max sprocket size is sometimes stamped on the mech...and is usually a bit conservative.
PaulD