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Can anyone tell me the difference between Shimano HG51 and HG50 8-Speed Cassettes?

I can't find any information online about this so was hoping someone might be able to help me.

I am looking to replace the HG50 8-Speed Cassettes on my mountain bike but searching online there don't seem to be many about and they are more expensive than the HG51 which seem to be in higher supply as well. So I'm thinking of buying the HG51 to make some savings but not sure if its compatible, I'm thinking it might need a different chain to the HG50 or something?

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The only difference is the model number, and the year of the design. They are fully compatible, assuming the number of gears is the same, and that you buy the appropriate match to your bike. The road version is Tiagra 8 speed. The mountain/hybrid version is Alivio 8 speed.

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  • Excellent thanks for your reply... I knew I would be able to find help here... I love stack exchange!!! If only that small bit of very important information was available on the Shimano website. I am going to replace the Alivio 8 speed HG50 with the HG51, just to confirm a Shimano HG50 Chain is compatible with the HG51 casette?
    – i-am-andy
    Mar 4, 2012 at 19:43
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    The chain is compatible, yes. Also, the lock ring tool is the same.
    – zenbike
    Mar 5, 2012 at 6:16
  • Brilliant thanks again... ordered up the parts and looking forward to replacing my first casette and chain.. looks like a fairly simple procedure
    – i-am-andy
    Mar 5, 2012 at 22:50
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I have a Shimano FH-RM30 rear hub, and I just swapped a CS-HG50-8 11-34t with a CS-HG51-8 11-32t and the new worked just fine as others mentioned.

Some small design differences I noted:

  • there is no 11-34t on the 51, max is 11-32
  • on the 50, the last two cassette gears came off, but on the 51 only one did. This manual seems to say that the 50 should have a single separate gear as well, so not sure what was going on.

Some related pages of the official website:

Here's a side by side before I put the new one on, 50 on left and 51 on right:

enter image description here

enter image description here

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The chain is compatible. However unless your chain has really minimal wear replace it with the new cassette. If the chain has anymore than minimal wear it will wear the new cassette prematurely. Oddly the Shimano website shows two different lock ring tools TL-LR15 for the HG51 and TL-HG16 for the HG50. It is possible that the tool part number was not updated in the spec sheets. Just keep this in mind before you start the swap.

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  • Thanks... thats an interesting spot... I have ordered this Ice Toolz casette lock ring tool - bit.ly/wXxqJu. This is the first time I have attempted any kind of bike maintenance at this sort of level so I'm hoping the tool will fit. If it doesn't I will report back and probably conceed defeat and take the bike to a professional to do a proper job. The only reason I am trying it is I have recently bought a knackered old mountain bike so thought I would attempt to do the work myself in a effort to pick up a few more skills.
    – i-am-andy
    Mar 5, 2012 at 22:59
  • Forgot to say I am replacing both chain (Shimano HG50 Chain) and casette (Shimano HG51 8-Speed Cassette)
    – i-am-andy
    Mar 5, 2012 at 23:01
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    Some times doing it yourself twice is still cheaper than paying someone.And you can never have too many tools.Just a reminder if the cassette is not a exact match for tooth count your new chain may have be longer or shorter.
    – mikes
    Mar 5, 2012 at 23:48
  • I have made sure its the same tooth count 11 - 30
    – i-am-andy
    Mar 6, 2012 at 21:46
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    Just wanted to confirm that everything has worked perfectly. Changing the casette was very easy, the lock ring tool worked fine, also changing the chain was a lot easier than I thought it was going to be. I can also confirm that the HG51 Casette is compatible with the HG50 chain. I now have great shifting gears and the knowledge of how to do this in the future. To any other first timers doing this procedure I would say just go for it... its really not that hard and there are loads of good tutorials on youtube if you do get stuck.
    – i-am-andy
    Mar 18, 2012 at 22:17

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