Maybe this will help somebody. I don't work on many Shimano spinning reels, but just finished servicing a Sienna 4000FD. After reassembly, it felt like the drive train was catching a bit twice during every complete revolution. Everything else looked and felt fine. Puzzled, I found a schematic online and noticed an insert, an expanded block diagram, showing the arrows on the drive gear must align with the peg on the oscillating gear. Obviously, the arrows on the gear didn't catch my attention the first time haha. Lined up the gears and smooth as butta. Guess I needed more Shimano practice anyways.