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I will install a Deore RD-M6000 derailleur on a bike that already has its cassette. The Shimano RD specification claims for a 32 to 36 largest sprocket, but my cassette (almost new) is a 11-28.

Is it a problem to use this slightly smaller sprocket?

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It will be outside the official guaranteed-to-work specifications published by Shimanro.

Effectively your 28t max cog will be a little smaller than the 32t cog, so the top jockey wheel will sit a little bit further away than ideal. You may be able to fiddle this with the B tension screw, but that could give you problems elsewhere.

Also note that it will be easier for this combination to throw the chain over the cassette and into the wheel. I'd suggest you be very miserly adjusting the low-limit screw, and only give yourself just-enough to get the low cog (ie the biggest, 28t one)

However we know that Shimano's specs are notoriously conservative, and going one or two over/under is generally okay. You're going 4-under, which is probably okay, but the proof's in the shifting.

I'd try it, and I'd also save my spare chain offcuts. If it doesn't work, get a larger cassette too.

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    I’ve run a 12-25 cassette (albeit on a trainer) with a long cage road RD (rated for at least a 28t big cog). I’m not sure the risk is throwing the chain, but the shifting is definitely not as good as it should be.
    – Weiwen Ng
    Nov 2 '21 at 15:53
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    @WeiwenNg There's more space between top jockey wheel and the 28t cog, so the chain has more space to move through and drop over the top.
    – Criggie
    Nov 2 '21 at 18:23
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    OK, I purchased a 11-36 cassette (not that easy to find today). I think I will be able to reuse my 28t max on a future project with road components.
    – rvil76
    Nov 3 '21 at 7:04
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The guide pulley will be further than ideal from the cogs. Shifting will be laggy and with narrower than usual margin for error in adjustment when everything is new and in good condition, but will function. The problem with doing this and the reason it's not just perfectionism is that once the parts have normal wear and tear (pivot slop, cable friction, some amount of hanger misalignment), the extra b-gap can make performance pretty bad and finicky or impossible to get a good adjustment on. As Criggie points out, it will also make it some amount easier to throw the chain into the spokes. I wouldn't be happy setting a bike up this way.

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