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...I have cleaned literally hundreds of rear derailleurs in an ultrasonic. It works fine, but you'll need to re-lubricate the pivot points and take off the outer cage plate and the pulley wheels to do a good job. If it's a Campagnolo New Record, when you get all the crud off, the pulley wheels are probably cracked. This is because of shrinkage of the plastic they used, not a result of using the ultrasonic. You can still use them that way, but most people replace the wheels with something newer that works.