Another important step in putting the engine together is to degree the camshaft. The cam determines when the valves open and close, thereby controlling when the fuel/air mixture goes into the chamber, and when the spent exhaust gasses go out. In the 4.0L, the distributor is also spun off the cam and tells the computer when to fire the spark plug, so it also effects when the mixture burns. Get it in there too early (lighting the flame when the piston is still coming up) and you can damage parts. Get in too late (when the piston has already started going back down) and you won't be making the power you could. Degreeing it is the only way to make sure you are dead-on. All of 505's engines are degreed before getting the head bolted on, and ours was no exception.