I finally decided to do something about my overhead vent covers.  I have one in unbroken condition, and the other three are/were definitely missing sections.  They are an incredible PITA to remove, and this one took a beating getting it out.  You need to get a 1/4" lock-nut off from the top of the knurled knob.  I put a wrench up through the vent hole and could turn it a little at a time.  There has to be a better way!  Does anyone have a special tool to accomplish this?  It needs to ratchet and