I have the NH-C12P and all fans go on the same.
Just hook the clips through the body of the HSF, then pop the protrusions of the clips into the holes.
As the clips only go through the first set of holes, it doesn't matter if it's a 25 or 38mm fan, they all fit.
On another note, if and when you do push/pull, remember to have the fans pointing the same way.
Just hook the clips through the body of the HSF, then pop the protrusions of the clips into the holes.
As the clips only go through the first set of holes, it doesn't matter if it's a 25 or 38mm fan, they all fit.

On another note, if and when you do push/pull, remember to have the fans pointing the same way.