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I'm pretty sure that the rear derailleur is not tensioning the chain properly. The pedal power is making the chain to bounce on the drivetrain, and causing the chain to fall. The crack sound - I suspect that the bouncing chain is going back to the chainring and catching the teeth. I'm surprised that the bike shop is not catching the problem.


Given that all other things, e.g. type/diameter of the axle, are the same, the difference of two teeth on the middle chainring do not present a problem.


The FC6603 seems to use a standard 130 mm BCD 52t chainring for the outer ring, unlike the special ones used on some other Ultegra/Dura-ace cranksets. You can buy such rings from most major brands. Look for ones marked 10 speed or 11 speed.

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