Hole cutting for 120mm fans

when cutting these types of holes, does 120mm mean from blade tip to blade tip, or does it mean the diameter of the fan shroud? I need to know beforehand so I know if 4.5 in hole saws will work or if a 5" is needed.

It's probably much easier if you simply mark where the screws are going to be then give that a small gap in a square and drill out holes for ventilation rather than cutting a piece off a panel

If I am understanding you right, that would normally be fine, But hole drilling would require grinding to make a flat surface on one side or the other to make it flush. Cutting a hole would not create that. The way I want to do this also create a way to top mount a rad later.

It shouldn't be too bad if you clamp it down to a piece of wood to drill into it.

Otherwise not sure how stable it would be to add weight to the structure after a large piece is missing.

to answer your question It's measured from the outside of the shroud, if you haven't purchased fans yet fractal design makes 140mm fans that can convert into 120mm essentially with a bit of overlap normally


When I was building fan mounts for my own PC (link), I drilled the 4 holes first, placed the fans in position and used a sharp pen to trace the inner contours of the fan opening using the fan itself. (you can see that contour in the bottom right of the 2nd picture on the linked topic).
Then I did them by hand with a coping saw. IIRC the holes on mine were 117mm in diameter. You could of course draw diagonal lines to get your center, then drill the holes with your hole saw and file a bit if needed.

Going slightly too big isn't a problem, the fan mounts can hold the fan just fine without needing to have it touching the metal all around the edge. It just isn't a pretty sight IMO.
Going too small however will generate lots of turbulence near the edge of the fan and is likely to negatively impact the fan's performance and acoustics (turbulence usually generates wind noise).
If you drill with that 4.5", you'll need to do some filing to get a nice smooth transition from fan to case. Seeing as 4.5" is 114mm and most fans' inner edges are in the 116-118mm diameter range, that can be up to 2mm all around.

I may wind up going too small and trying a bit of silicon or weatherstripping to take care of the noise if it is too bad, but I am not like most, I do not really get bothered by fan noise until it sounds like it is in A-10 thunderbolt zones. Awesome build, Too bad my NZXT H230 does not have that kind of room. I did however think about putting 2 more 120mms in the front door in front of the standard 2x120mm. But I am working on the top for now. Thanks for sharing the build!