Originally posted by compy
just get a 120mm drill bit and drill...
I belive you mean a 120mm "hole cutter"
Adjustable Hole Cutter
NOT a drill bit:
If you were actually find a 120mm drill bit at the closest industrial
machine shop/supply store, it would require at least a 2-4 inch chuck, a power-device, (such as a drill-press) that accepts that kind of chuck... the bit alone (used) could be easily hundreds of dollars, and the chuck could run right up there with it. (Assuming you actually FIND them)
Just thought I'd say that, so noone goes to their local chain-hardware store looking for a 120mm (~5 inch) drill bit, and get laughed at.
EDIT: To find the airflow direction, just look at the blades - use your knowledge of physics and air resistance/friction to figure it out. I never really look at the arrows. If that doesnt help, the air always flows on the side of the motor supports (one side is completely open, and the other has four plastic supports in an X or + shape.)