It Holds a 40mm fan for cooling the extruder on the new Simple(1405). I have had many problems with my extruder softening the filament and creating a jam so i designed this before the upgrade to the new 1405 Simple. Designed to swing out of the way for loading the extruder or any other reason. Pictured with Lemm's Printrbot fan cover. Found here: http://www.thingiverse.com/thing:261115Assembled with a 22mm or longer metric bolt. a nylon washer and a lock nut. Assemble with a nylon washer between the two parts. Use a lock nut if available. The 16mm screws still work to install it on the side.