Summary This is a 40 mm fan cover or guard that is designed differently than most. I needed it on my Prusa i3 so I could protect that fan on the J-Head duct, and also have a place to apply warning stickers. If fastened with 3 mm cap screws, the tops of the screws will sink into the recesses and not protrude from the flat top. It can to be useful to protect a fan placed on the top of something. The cover tends to prevent small objects from falling through the openings of a typical fan guard.