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Does anyone know if an Ultegra 6800 left crank arm will work on an Ultegra R8000 crank? I'm looking at upgrading my road bike, and I'd like to move my current crank-arm power meter over to the new bike rather than spending even more money. Thanks!

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    So the power meter is the left crank arm, right? – Argenti Apparatus Jan 24 '18 at 1:52
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    @ArgentiApparatus no not the right, the left! – Criggie Jan 24 '18 at 2:10
  • That's correct Argenti. I'm unaware of any crank-based power meters that are right-only. They all are either dual-sided or left-only. – KevinC Jan 25 '18 at 1:06
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    You can use the 6800 left arm provided it has the same length as the R8000 right arm! Although it might look a bit odd from the design but that is a matter of taste only. – Carel Jan 25 '18 at 9:13
  • Oh man, I didn't think about length. The size on the new bike is 172.5, whereas the old one is 175. I guess that makes this question moot! – KevinC Jan 25 '18 at 18:39
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Yes

The website for the Stages Power Meter mounted to a Shimano Dura-Ace R9100 non-drive-side crank arm says:

Crank Compatibility: All Shimano Hollowtech II road cranks

This also agrees with Rider_X’s comment and my own guess that they should all be compatible.

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The only difference is that the crank has moved the small ring inboard by 0.4mm and kept the large ring in the original position.

and

(are) 6800 chainrings compatible with r8000 cranks? Answered as no

According to http://forums.roadbikereview.com/components-wrenching/new-ultegra-8000-compatibility-6800-a-360975.html

Thing is, your power meter is in the left crank arm only, so the only interface that matters is the crank-to-BB. Shimano's own documentation is lacking - the best I could find was 2017-2018_Compatibility_v023_en.pdf which is woolly at best.

The other resource is http://www.cyclingweekly.com/news/product-news/shimano-ultegra-8000-series-vs-ultegra-6800-series-key-differences-334205 which compares the differnces between the two, but still doensn't show the crank to BB interface in a picture.

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    The left hollow tech crank arms have been compatible across all the product tiers and across many years. I would be surprised if R800 changed that. – Rider_X Jan 24 '18 at 3:31
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    @Rider_X that's an answer - you should post it as one. Its also better than my collection of misc results. – Criggie Jan 24 '18 at 10:25
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All hollowtech 2 shafts are the same to the non-drive side arm to my knowledge. I have seen 105, Ultegra and Dura-Ace of both the 2017 gen and the 2019 gen and they're all identical.

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