Anyone know how to install one of these on the wr450? It looks like the impeller just threads on, but when I try to take the old one off it just spins. I bought it off a friend of mine for cheap, but he never put it on it just sat in his garage. Any thoughts? if I have to go through the top end im just going to sell the damn thing becaused there is no way im going to do that. Let me know, thanks...