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I'm using a Shimano 9100 crankset currently, with 53/39 9100 series chainrings, but on my last bike I had 53/38 and I loved that combination. I have a 38 tooth chainring to put on, but it doesnt currently fit. While the change from 9000 to 9100 didn't change the chainring bolt position at all (I've checked and they line up perfectly) or the interface with the crank on three of the four arms, the fourth has been trimmed down, to make room for some more material on the crankset. This seems to have been done entirely to force consumers to purchase new chainrings, but they no longer make a 38 tooth in the current series of Dura Ace. Would it be safe to modify my chainring and where would I find someone who would do this?

  • Can you expand on the change to one of the attachment points? It's not clear what 'trimmed down' means. – Argenti Apparatus Nov 13 '18 at 12:54
  • It's much easier to show the difference on the outer chainring, but the same change was made on the inner ring. I've tried to highlight the issue on these pictures: imgur.com/a/38sdE5q – Carbon side up Nov 13 '18 at 13:02
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    Picture 1 is the crankset that I'm using, with the correct 9100 chainring fitted. Picture 2 is the same, but just highlighting the part which wont fit. Picture 3 is a 9100 chainring on top of a 9000 chainring. In picture 4, I've highlighted the extra material on the 9000 chainring that isn't present on the 9100 series. Does that make it clear? – Carbon side up Nov 13 '18 at 13:03
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Here's the difference in the chainrings.

enter image description here enter image description here

Shimano might have modified one attachment point to ensure that the ring is oriented correctly. There does not seem to be any differences in the teeth though.

As Shimano removed some material, it's obvious that you can. It would be possible to simply mark out the profile of the modified arm, hand file away the extra material and test fit as you get close to the line. Filing with reasonable precision is not as easy as it looks but there are many videos online that show you how it's done.

If you need to get someone to do it for you, look up local machining shops.

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    A stable vise, soft jaw protectors to avoid marking the ring as well as fine file will be needed. Slow work without hurry and frequent checks of the fitting are essential. – Carel Nov 13 '18 at 17:15
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    @Carel, agree with the above but use of a rough file to remove most of the material will be necessary so the job does not take forever. Then switch to medium, then fine for final fitting. I'd look at the clearance between the other arms and crank and build that into the final fit, to avoid creaks and allow for thermal expansion. – Argenti Apparatus Nov 13 '18 at 17:19
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    A rough file might remove material rather too quickly since the rings are quite thin.The most difficult part will be to avoid rounding the edge and to keep it a right angle to the plane of the ring. I'd start by drawing the outline of the required shape on the 9000 ring. – Carel Nov 14 '18 at 13:51

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