If your visors keep falling down even though you have tightened the tension screw, then the shaft on your visor mount has come loose. You don't need to buy a new one!!!
Here's how I fixed mine.
Just drill near the edge of the shaft housing all the way through the shaft and install a small roll pin. SIMPLE AND EFFECTIVE.
Just like it should have been done to begin with.

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86 Fiero GT

That could be taken to the next step by inserting one of the newer model visor posts from a different car; the kind that slide back and forth in addition to flipping up and down.

or try Green formula Locktite.
Green wicks into parts already assemble.
example Loctite 37423 290 Green Penetrating... amazon

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)