I was given a 2nd hand bike with a rusty chain. The cassette looked fine so I replaced just the chain.

The new chain kept slipping on small cog-all the others fine. Cable adjustment etc made no difference.

When I put the old chain back on there was no slippage.

Could it be an incompatible chain

  • 4
    What cassette is on the bike, and what chain did you put on? If they are compatible, the cassette can be worn.
    – Christine
    Jul 31, 2022 at 13:42
  • 1
    Did you properly adjust the length of the new chain? Jul 31, 2022 at 15:58
  • 1
    the cassette is six speed-no front cog -.The new chain was advertised for 18 speed i cut it exact size of the one i took off Jul 31, 2022 at 18:45
  • 1
    If the old chain was stretched out, as is likely the case, then the new chain might be too long.
    – Gary E
    Jul 31, 2022 at 18:51
  • 1
    sorry when i say exact size i mean exact number of links not actual length Jul 31, 2022 at 18:56

1 Answer 1


The cassette looked fine

A cassette almost always looks fine, even if it is quite worn. The relative chain difference from fresh new to completely stretched out is under 1% of its length. Human eye cannot catch such small difference.

A cassette that has been used with an overly worn out chain will also become worn out, but the signs will be even less noticeable. At least, it takes a trained eye to estimate the wear. For most people, just by looking at cogs and teeth, it is usually impossible to predict if they will be skipping with a new chain, or whether it will be fine. There are (almost) no tools to measure wear of cassettes (but there are for chains).

Provided that it is indeed a wear problem and not a compatibility issue, you have these options.

  1. Replace the cassette and use the new chain.
  2. Replace only the worn out cogs. It is possible with some cassette designs, especially for older products. More recent cassettes often come as monolithic blocks where not all individual cogs are replaceable.
  3. Continue using the old chain and put up with the wear it creates until you get to replace everything.
  4. Try to "restore" the worn-out cogs by filing its teeth. The technique is described here. I tried it once, and so far I am content with the results. Obviously, there is no guarantee that it will work for you, and that an attempt to modify the cassette won't make things worse or even dangerous to ride.
  • I tend to go with #3 and ride a complete transmission into the ground. The chainrings generally are okay but the cassette and chain can wear each other out until shifting suffers, or they slip too much. Then change both at the same time.
    – Criggie
    Jul 31, 2022 at 19:20
  • yes i thin i'll leave it alone for now .I cant understand how the chain got stretched and cassette seems like new Jul 31, 2022 at 19:35
  • @KeithBrufordBennett that's the point - 1% wear on a cassette where the teeth are the width of a fingertip apart is about 0.15mm difference, impossible to see. Try using one fingertip as a gauge - press it in between the teeth on your least worn cog. Then compare that fit with the gears that slip the most.
    – Criggie
    Jul 31, 2022 at 22:41
  • @KeithBrufordBennett the other thing to try is look at the curve between each tooth on the cassette. It should be a smooth single circular line, and not change in radius from one peak to the next. It should be a smooth consistent U and not a ` __|` (I should get photos and make this a proper answer)
    – Criggie
    Jul 31, 2022 at 22:43

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