I have looked at the Illustrated Parts List for the 18166, which may or may not be the same as yours. Are you talking about a problem with the dual pulley, with an upper part which is driven by a belt from the engine, and the lower part which drives both cutters through a serpentine arrangement? Here is the parts diagram:
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I think we are going to need some pictures of your machine, so we can both speak the same language on this.