Crank Arm Compatibility - Shimano Ultegra 6800 and R8000 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!
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.