I would guess that they are #6-32, but you can always just take a screw with you. I always go to ACE hardware for screws and such as Home Depot and Lowe's are worthless for this kind of thing as you can never find help. Just make sure your screws aren't so long that they hit the rad when you tighten down the fans. 1 1/4" long are all you need for 25mm fans, or maybe 1 1/2" for 38mm thick fans.