I'm trying to put a Boyesen Water Pump Cover (which has a new impeller 75% larger blades than stock one) onto my 2005 WR450 to help it run cooler. The Yamaha owner's manual has a wonderful set of instructions which says "Remove the impeller" and shows a hand holding the old impeller and impeller shaft. The Boyesen manual says "Remove the stock impeller". Nobody provides details about how the removal should actually take place. How do you get the old impeller out of there? Trying to turn the impeller wheel counterclockwise to unscrew it only turns the entire engine over (and I don't want to break anything). Any great words of wisdom from someone whose done this themselves?