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