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I'm considering swapping the inner ring on my SLX double chainset (M665) from 22T to 28T. I noticed that the newer M675 has a 40/28 option, so I figured that the 28T chainring from this set-up would work well on my slightly older chainset.

However, the Shimano schematic for the M675 chainset suggests that the chainrings aren't backwards compatible with M665. But I can't see how a flat inside ring with no pick-up points machined into it wouldn't be compatible, providing the bolt patterns match up (which they do, I think).

Have I missed something?

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  • I think there are some pins which portrude a bit on the M675 chainring which you'd have to grind off which aren't present on the M665.
    – Batman
    Dec 30, 2014 at 13:53

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I can't see how a flat inside ring with no pick-up points machined into it wouldn't be compatible, providing the bolt patterns match up (which they do, I think).

The bolt patterns are compatible.

There are two reasons why the chainrings still may be counted as non-compatible:

  1. Not all Shimano chainsets have identical fitting of the chainring, even if the bolt circle diameter is the same. Sometimes a chainring has to be slightly filed on the internal diameter. I tend to think that this clearly intentional incompatibility is not introduced without a reason and there is some other subtle difference.

    In practice: I used some "force-filed" chainrings and do not remember noticing poor shifting.

  2. 28T chainring will not be "hyperglide-compatible" with your larger chainring, whatever size it is. The latter is designed especially for upshifting from the 22T. Being shifted up from the (rightly positioned) 22T, the chain links hit the shifting pins and dents so that the chain engages properly. Being upshifted from another chainring, the chain links would not be correctly positioned (except if by luck).

    This is the reason why, for example, the Shimano PDF which you linked lists two different 38T chainrings: "38T-AM" and "38T-AK". The former is "hyperglide-compatible" with 24T-AM, the latter with 26T-AK.

    In practice: Some people would say that the difference is again not noticeable; here I disagree. IMO the front shifting degrades seriously.

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  • Thanks – this is really helpful. Another question, if you don't mind: according to the tech docs, the M665 chainset isn't 10-speed compatible (although the M660 is). Would this be down to the chainrings being too wide?
    – Will Vousden
    Jan 7, 2015 at 11:57
  • Actually, it seems more sensible to post this as a new question: bicycles.stackexchange.com/questions/28447/…
    – Will Vousden
    Jan 7, 2015 at 12:56

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