Originally Posted by icemilkcoffee

The problem is the RSX group uses STI shifting. The 8 speed crankset, although using the same width chain as the 7 speed, has different chain ring spacing than the 7 speed crankset if I recall. Furthermore the Ultegra 6600 crank is meant for a 10 speed drivetrain!

To the OP- to the best of my knowledge, there is no Hollowtech II crankset available for that 7 speed drivetrain. You could try a Claris crankset meant for an 8 speed drivetrain. You need to swap out the left brifter though to get the correct click spacing.