I currently have a Specialized Secteur with a Shimano Claris 2400 crankset (compact). I'd like to buy a Stages power meter but they do not sell Claris crank arms.
Can I simply put a 105 crank arm on a Claris crankset?
Not a modern 105 - Claris cranksets are octalink, 105 cranksets are hollowtech.
After a few years:
Can I simply put a 105 crank arm on a Claris crankset? - Yes, Claris R2000 is a Hollowtech II crankset and the left arm is independent of the right one, it just attaches to the same kind of splined spindle.
Be careful, even in new cheap bikes you can still find Sora R3000 groupsets with the older FC-R345 Octalink cranks instead of the modern FC-R3000. Something like that could happen with a Claris R2000 groupset with older Octalink cranks (FC-RS200,FC-2450,FC-2403). Alas, they even sell new bikes with the 105 groupset but with square-tapper cranks (e.g., GT GTr Elite 2019).
The title question: Can I fit a 105 crank arm (stages) to a Claris 2400 crankset?
No, the old 2400 is Octalink and only some old 105 types could be used with it, It likely would not solve the problem with the powermeter.
