self tapping screw into pre-existing hole (need help with sizing)

[whiskywizard]

I am attaching some 3/4 plywood to an angle iron frame (deer stand floor) As an afterthought, I'm planning to use self tapping screws that will counter-sink and be flush with the plywood. I already have holes drilled into the angle iron. I was planning to use drywall screws screwed in from the bottom (just long enough that they would not completely penetrate the plywood. Anyway, the small threads did not really catch the wood as good as I wanted, so I decided to remove the drywall screws and drill up through the plywood so that I could see where to go with self tapping screws that will go in from the wood side and catch the angle iron. Here is my delima. The holes in the angle iron (3/8 " angle iron) are 5/32. Most of the self tapping screws with heads that will counter-sink are listed as #4 or #6 etc. Here is my question:

What size "self tapping" screw will I need to get to fully utilize an already existing 5/32 hole in the angle iron?

What you want are weather resistant wood-to-metal flatheads. Check here
McMaster-Carr

The 5/32 pilot holes you've drilled already weren't really necessary but they also won't hurt. The minimum size you can use now is the 10-24 x 1 7/16. Part number 94054A301
You can go longer too (I would) and use 11-16?3 Drive. They would be part number 94054A320

 

[Screwlady]

Have you thought of Reamer Teks? They are designed to go from wood to metal. The 'wings' clean the threading through the wood and then break away when they hit the metal. Designed to be used in truck beds.

 

[billybobjimmyjoejack]

Reamer Teks are great screws for your application but it sounds like you don't need very many. I would use a flat head machine screw with a nut on the bottom, a #6 is the largest that would fit into a 5/32" hole and might be a little small. I would recommend that you make the hole larger to accommodate a larger screw like a 3/16" hole for a #10 screw or a 1/4" hole for a 1/4"-20 bolt then you would have more choices and the hardware is more readily available.