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Today I have replaced my chain with a kmc z8s. The rear cassette is Shimano acera 8 speed , and is not worn yet , so everything is OK with the cassette. But the problem is that the chain doesn't fit the cassette properly ,when I apply more power on the pedals the chain starts sliding over the cassette (The lenght of the chain is OK too). What is the solution of this problem ?

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    Are you sure you threaded the chain through the rear derailer properly? Mar 19 '14 at 15:35
  • Yes everything is OK . The only problem is the chain Mar 19 '14 at 15:39
  • Does it happen on all of the gears? Are you positive it is an 8 speed chain?
    – canadmos
    Mar 19 '14 at 15:59
  • Is your Acera a Hyperglide (HG) or an Interglide (IG) cassette? There can be compatibility problems causing jumping if you mix the two and run an HG chain on an IG cassette or vice versa.
    – JohnP
    Mar 19 '14 at 18:03
  • Are you sure you cut the chain to be exactly the same number of links (not the same physical length) as the old one? Mar 19 '14 at 20:08
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  1. The cassette really is worn. It takes something of a trained eye to spot a too-worn cassette, and often when you switch to a new chain the cassette will skip worse than it did with the old, worn chain.
  2. The chain is the wrong one for the cassette. Chain/cassette compatibility is an area where there are no nice general rules -- you have to make sure that the chain you get is compatible with your specific cassette. This includes having the correct speed pairing (e.g. 8-speed chain for 8-speed cassette).
  3. The chain was cut to the wrong length. A chain is always delivered with 8-10 inches of extra chain. You must shorten it to the appropriate length, using a "chain breaker". Some people just wrap the chain around the largest sprockets and eyeball it, but if you haven't developed that skill the best thing to do is to lay old and new chain side-by-side and carefully match up links along the length so that there are the same number of links in the new chain as the old. (Note that if you just match length the new chain will likely end up a couple of inches too long.)
  4. The chain was threaded through the rear derailer incorrectly. It's kind of hard to mess up on the front, but the threading through the rear derailer is not intuitive, and it's possible to muck it up.
  5. The derailer is out of adjustment or has been bent somehow.
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it sounds like a derailleur adjustment issue. although HG vs IG compatibility issue could be something to be concerned about. it seems like the tension on the derailleur cable needs to be adjusted, and the stops too possibly. here is an article about how to go about derailleur adjustments.

http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

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