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So I've never changed a chainring before. But the first step is finding the right one.

I have a Cube Hyde SL 2012 . It describes the crankset as

Shimano Deore XT FC-M785 Hollowtech II 42x26T., 175mm, integrated BB

By which I assume the 42 refers to the number of teeth on the outer ring - which is the one I need to change.

The closest thing I can find is this (M780):

But a) it suggests it's a triple chainring, and b) in the Q&A on that site someone has asked

"I'd like to swap out the 40t large ring on my Shimano XT M785 10 Speed Double Chainset with this 42t ring, would that work?"

With the answer

The chainrings for the M785 crankset have a specific design and unfortunately are not listed as having a compatible interchangeable option.

So.. err.. what now?

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You are correct that the 42x26 is telling you that the large ring has 42 teeth (and the small 26).

I did a search on "M785 replacement chainring" and found a number of sources (but nothing a Chain Reaction). Looking at what I found it appears that the options for the large ring are 38 and 40 teeth. So…

Either Shimano has changed the specs on the M785 and the 42 tooth ring is no longer available, or there is s typo on the Cube site. I'd say it's worth looking at your bike to see if the tooth count is stamped on the chainring or, failing that, count the teeth.

If it is a 42 tooth ring (and you want the higher gears) it might be worth going to your LBS to see if they can order the 42 tooth ring, or else look for NOS (new old stock) rings on places like Ebay.

  • So silly me for trusting the documentation. Upon actually counting, it's a 38t, not a 42 at all! – user340535 Jul 30 '15 at 14:45
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Firstly, no you can't use a triple chainring on a double crankset.

Second I'd count the number of teeth on the actual chain ring.

I don't recall, and was unable to find any evidence of, a 42T XT chainring. So would question if it was a printing mistake. It is possible it was made available as OEM only, but I would have thought it would be given a different product code with the XT branding removed which Shimano did for a number of cranksets a few years back.

The FC M785 came in 3 configurations (but more may have been possible as an OEM product).

  • 38-24
  • 38-26
  • 40-28

A 42T was available for the FC M780 triple chain ring but not in the double.

When selecting Shimano chainrings there are a few things to consider:

  • Number of teeth
  • BCD -Bolt Circle Diameter is the diameter of the circle that goes through the center of all of the bolts on your chainring
  • The shifting combination code (not sure of the proper name) - this is AM/AJ/AK for the FC M785 and is used to match rings to ensure ramps and pins align for shifting, not a deal breaker but ideal.
  • If it is for a triple, double or single crankset - this will impact the offset and profile of the chainring, to make sure it aligns and fits with the crank, other rings and chainline.

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