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First two pics show the standard N-Fab pinch weld mounting. Per their instructions, the only things supporting the steps are six bolts each side, through the pinch welds. I substituted 3/8" stainless bolts for the supplied hardware. Third pic shows the rivnuts/nutserts installed into the box section of the rockers, taken from under the truck. These are heavy duty, cad plated, knurled steel serts using a 25/64" hole. Fourth pic shows the reinstalled N-Fab steps, with 1/4"x20 Grade 8 cap screws into the rivnuts, with anti-seize on the threads because they will have to come out sometime. I added short compression sleeves behind the steel plates where the recesses are formed into the rocker. Sleeves are just a large I.D. nut of appropriate thickness, with the 1/4"x20 slipped loosely through that nut.
The standard N-Fab mount is fine as long as the only load applied to the steps is the weight of drivers and passengers getting into/out of the truck, because their steel plates rotate in slightly to bear against the inside of the rockers when downforce is applied to the steps. But, if you apply an upward load from under the steps (like getting stuck in deep sand or hitting a stump or rock with the step), the whole step assembly tends to move up and that rotates the steel plate away from the inside of the rocker, and that's how I had bent my pinch welds. The 1/4"x20 bolts hold the plate flush against the rocker to eliminate rotation and keep the pinch weld from bending. I used 14"x20 because those were the biggest steel rivnuts I could find, and I used Grade 8 because the bolts are in both tension and shear and G-8 is a lot stronger than stainless. Rivnut placement is not scientific, but is a function of where I could actually fit my drill and nutsetter. Installation is now noticeably more rigid when stepping into and out of the truck. Steel brackets are shiny because I cleaned them up and shot with engine enamel while they were off the truck. The N-Fabs were/are full of rust, especially inside the main tube, so they probably won't last as long as the truck does. Called N-Fab and they told me that the tube interiors are not coated with anything and that their warranty applies only to the outside powdercoating