I have noticed that some engine coolingfans (especialy olderones with five blades) have two blades closer to each other instead of being equaly spaced. Does anyone know why? How would it affect the engine if one of these lopsided fans was replaced by a lighter evenly spaced fan such as a flex-a-lite?

 

As long as they're properly balanced, everything is fine. The offset blades are balanced, and allow for access in the engine bay through the fan for tightening belts.