Well I sold my bike and I'm ready for a new one. I've been browsing adds and a lot of the '06 yz450f's seem to have work done to the engine such as rebuilds, why? My 10 year old yz400f ran perfect and strong, why would a '06 need engine work? Here is one I'm considering but the engine work scares me and whats the purpose of heavy duty clutch springs? Will that make the clutch lever stiffer? Pending the advice from you all I may call on it. The suspension is already setup for my weight which is nice. When searching on here I'm really not seeing any engine problems with the yz450f's, everyone seems to feel they are bullet proof.