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I now have 404 cfm being drawn into the case from the side panel plus another 127.5 cfm that will be drawn in through the front 80mms. Exhausting out the top is 261 cfm plus 80 cfm through the front blower and around 36cfm through the power supply. That is 531.5cfm intake and 377cfm exhaust. Not quite balanced. So one more 120mm YS-Tech (131.5cfm) will be installed in the back for a total of 510.5cfm exhaust.
Only problem is someone has already cut a 92mm blowhole here. Without being able to use a pilot hole the big holesaw will jump all over the place making a mess and probably never cutting the larger hole.
So the first thing to do is cut the formed grill off
Once it is out of the way, the fan is centered and the four mounting holes are drilled.
The plan here is to put another piece of metal behind the existing 92mm hole so the arbour will have something to keep the saw in place. I cut a square from an old piece of shelf using my mini-grinder (a very handy tool for big cutting jobs).
Now four more mounting holes are drilled in this piece of metal and the center can be found and drilled.
So now when I bolt this plate to the case I have a stable point for my arbour.
And get another perfect 120mm hole.