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I have an Acera 3x9 mega9 setup (FC M391) on a 2015 Fuji flatbar. I am refurbing it for secondary use, trying to replace as little as possible.

My first question is if any 104bcd 9-speed chain ring will work? Existing is an SG-X U48. The triple config is 48-36-26. The 48 has the most wear.

Second is, since I am going to a new BB, chain & cassette, is a new crank a smarter bet? Rings seem pricey compared to older NOS.

I really have no reason to swap RD & FD, or shifters, by that time it will cost like a silk purse, but still be a bit of a sow.

Thanks for any comments.

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    Is the 48t chainring really worn down to the point where it needs replacement? Usually the big chainrings last a loooong time. Usually chances are good that any chainring will work, but sometimes they have silly ridges or notches which create incompatibility (not sure if this is the case for some of the 104mm BCD rings out there). New (but identical) crankset doesn’t really make sense since a cheap chainring is still cheaper and creates less waste.
    – Michael
    Jun 19, 2021 at 15:13
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    Some cyclists do looots of miles, and that tends to wear out stuff
    – ojs
    Jun 19, 2021 at 17:58
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    @Michael My large mountain chainrings always wore out before the middle ones. Especially so as the tooth counts dropped to 44 then 42 then 40. The middle rings were close behind. It was typical, however, that other parts of my MTBs failed or the bike moved on before replacement of the crankset was necessary.
    – Jeff
    Jun 19, 2021 at 23:40
  • The biggest ring may suffer more from strikes (rocks & logs)
    – Carel
    Jun 20, 2021 at 13:39

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The M391 crankset is a 104mm BCD, however, as noted in the comments, not any 4 bolt, 104 BCD chainring will be a direct swap fit. The interior portion of the chainring--the 4 areas with the bolt holes that communicate with the crank spider can have differing dimensions and shapes which interferes with the chainring fitting on to the spider. The holes obviously match up, but the shape or amount of material to the inside of the holes doesn't allow a correct fit on to the spider. Many that run into this problem get some use out of a grinder and take off material and shape the ends to get them to fit. The amount of material interfering is just a few millimeters at most typically.

Here is the Shimano Exploded View of the crankset in question. This has the part numbers of the specific chainrings. Also of note is that replacement rings can be obtained from the FC-M430 & FC-M431 cranks and these would fit without any modification. The Acera and equivalent cranksets are relatively inexpensive and since you are changing the cassette and chain (great idea as a worn chain will mess up the new crank and cassette if it can even run on the new stuff without slipping), it may be wise to get a whole new or gently used crankset. At this level, the cost of a new chainring, especially the large one, could be 50% or more of the cost of an entire, equivalent level crank set.

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