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Favero Assioma DUO Power Meter Pedals 
Garmin Rally RS200 Power Meter Pedals 
Wahoo POWRLINK ZERO Power Meter Pedals The only power meter that will bolt onto an FSA MegaExo crankset is a set of power meter pedals (as pedals are compatible with any crankset). There are no other power meters that will fit a MegaExo crankset. Stages Cycling used to make a MegaExo crank arm but discontinued it around 2017.
Please also note, if you decide on a new crankset-based power meter, we recommend changing your bottom bracket. MegaExo cranks use a spindle size that is 24.05 mm in diameter, vs. the more common 24 mm and 30 mm spindles. So your new power meter crankset will come with a 24 mm or 30 mm spindle and will need a bottom bracket to match.
I have a 2021 Giant Revolt Advanced 3 Gravel bike with one of above mentioned FSA Omega cranksets. The group set is a 2×10 Shimano GRX400. What would be the most economical solution get it a power meter on the bike if I didn’t want to buy power pedals?
Thanks,
-Ben
Hi Ben and thanks for the question,
Unfortunately, that FSA Crankset does not have a removeable spider and has a proprietary spindle. The least expensive non-pedal option will likely be replacing the complete crankset with something like this: https://powermetercity.com/product/power2max-ngeco-rotor-p2m-edition-24mm-gravel-power-meter-crankset/. It will also require chainrings and a bottom bracket.
Thanks,