The spacer should be whatever size that will put the fan blades 50% in and 50% out of the fan shroud.
Sorry to hijack this thread. Is this correct placement for all fan types? I believe that my fan is the original, I have no spacer and the leading edge of the fan (edge toward front of car) is even with the opening of the fan shroud (it's not recessed at all into the the shroud). I haven't had any cooling issues, but perhaps positioning the fan 50% into the shroud will provide better cooling?
