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my kids mtb on the smallest cassette cog the chain rubs on the next bigger cog creating a harshness while using this smallest cog. I was looking at it and the spacing between the two smallest gears seems to be smaller compared to the remaining gears on the cassette. My question is this normal? It is a Trek marlin and has never been apart since new. Thanks for any help in advance.

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    Can you post a photo? Or even a video? It is quite unlikely that the gap is smaller even though a missing spacer could cause that. It could just be a gear indexing issue. The gap certainly must be measured, e.g. by a caliper, cannot be just eyeballed. Commented Nov 27, 2023 at 14:50
  • Just one gear at the front? To eliminate thoughts about cross-chaining. I'd be looking at the limit screw as it's one end of the range
    – Chris H
    Commented Nov 27, 2023 at 17:19
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    The spacing between all cogs should be the same. Without photos, hard to say much more.
    – Paul H
    Commented Nov 27, 2023 at 17:25
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    Perhaps poor RD adjustment or slightly incompatible parts (chainline-issue) causing the front chainring being located to close to the frame.
    – WornChain
    Commented Dec 28, 2023 at 10:53

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The cogs should be evenly spaced. On a normal Shimano HyperGlide type cassette the smaller cogs should have plastic spacers between them. It’s possible whoever installed the cassette forgot the spacer or installed it in the wrong place.

There are two exceptions:

  • The smallest (11 teeth) cog usually has the spacer “integrated” into the cog, i.e. it’s thicker and doesn’t need one of the plastic spacers.
  • The two or three biggest sprockets are often riveted onto a “spider” and don’t need spacers either.

Make sure the cogs are evenly spaced and are not loose. If a spacer is missing altogether the lockring probably can’t press against the cogs and the cogs will be loose and have play.

cassette cogs and spacers

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    The perceived "spacing" issue could just be an optical illusion because of the tooth positions. When starting out road cycling, I also thought that my 11-speed cassette looks odd but that is just because the first 4 cogs are pretty much lined up while the others are offset and the gear jumps are higher (11-12-13-14-16-18-20-22-25-28-32T for my CS-R7000).
    – DoNuT
    Commented Nov 28, 2023 at 9:14
  • One that gets me on my recumbents is packed dirt between the smallest ~3 cogs. It makes things look weird, and eventually prevents the chain from seating well. Was hard to spot too.
    – Criggie
    Commented Nov 28, 2023 at 10:05

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