Bolted in. I used some 2.5" slotted screws at first to try to position the fan. I ended up using some 1.5" screws on the top, and the 2.5" and a 3" on the bottom I think. The small shroud around my fan had slots for the nuts, but I could only use 3/16", as 1/4" was too big and it was not worth the effort to grind them all down to fit.