well this is one thing unlike that water pump for the duratec that does last the life of the car. Excpecially when you concider most people keep a car to 110k and then ditch it. Well I changed my belt at 104k and the belt looked very good when compaired to the replacement one. only marks on it were from slapping the timing cover.
The main reason for changing it was the tensioner was making alot of noise. when we got it out it was going to crack in half soon. plus with the pullies worn down the belt was not centered on the cam gears.
job can be done with basic hand tools and some good know how. make sure you support the engine and index the cams when you pull the belt off. They will move alittle but but not much, was about one tooth of the gear. also make sure you get the spring needed to tension the belt, stupid one use $7 spring....anyway. take your time and check things out and it wasn't to bad. get the Ford Service manual and it outlines every step and had pictures and such, it is a great reference point
