Quote Originally Posted by Bas View Post
Well, in the end I had to use a drill and drill off the head. It was impossible to pull it out. After I removed the lid from the filter box I saw why: the thread isn't actually in the plastic of the filter box, but in a little metal oval-shaped cylinder-type thing. So I couldn't pull the screw out by stripping the threads, because those were metal too. and the screw didn't turn: it was still completely stuck in that little metal cylinder, and when I turned the screw, the cylinder would turn with it. I tried to dig it out with a screwdriver but it's impossible to get it out, so I just left it there. I refastened the other three screws and the lid still seems pretty secure on the filter box.
That's a threaded metal insert that is press fit into the plastic. It has broken loose due to the screw being seized in it, and it rotates when you put torque on it. You can either leave it that way, or rip it out by locking onto what remains of the headless screw and pulling up on it, or drill it out.

Considering that that screw became seized you might want to put some anti-seize compound on the other screws when reassembling the air box.