cos when you hold the cam's in the timed position they slip round under spring pressure , but like coley say's you'll see the slot's in the cam's oposite sides of the pulley's slide something of the right thikness into the slots so it rests on the cam cover mating surface of the head, it'll be obvious when by the engine

edit: they did put timing marks on it they also made it fool proof with a cam locking bar