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Old, did you see the pull out values I listed in my previous post ? Those are averages from real tests in similar thickness material. I am lookng at the manual again and will use 3/16" mild steel for the substrate,( what you are fastening to ) 3/16" is the closest thickness, the 10 gauge I used before is really for sheet metal, but is also close in thickness. If you use a Hilti Kwik Pro #12 screw wire gauge -14 thread pitch (#12-14) self drilling/tapping screw with a #3 point (point is sized according to the substrate fastening thickness) into 3/16" it will give an average ultimate pull out value of 2800 lbs, use a #1/4"-14 self drilling screw it will give you 3850 lbs average ultimate pull out value in 3/16" steel. These values will drop slightly for 1/8" steel, as it's 1/16" thinner, but still very strong for that application. If you are set on drilling and tapping, that thread pitch of 14 ought to work for a 1/4" dia. bolt, you just need to size it for tapping, what size tap for a 1/4"-14 bolt is what I'd be looking for, if 14 is not available 16 would work. The finer the thread, the thicker material you need to fasten too. Those self tappers will perform and eliminate tapping and reaching under or up to put nuts on and tighten, they even make em in stainless, but are also offered in 6 different finishes in carbon steel, corrosion resistance is good with the galvanized finishes and up using 48-144 hours salt spray tests which is fine for that application. Black oxide finish has the least corrosive resistance.
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