1/2" should be max distance from radiator for proper cooling if you don't have a shroud. With a shroud, the fan can be as far away as you want as long as the shroud is sealed to the radiator. The fan blades should be 1/2 in, 1/2 out of the shroud opening. You want the fan to pull air through the core of the radiator, not around it, 1/2" space between the two gives you some safety room yet still lets the fan do its job if you decide not to use a shroud.
All radiators will benefit from a shroud.
Joe
