I know the 6 holes on each side are supposed to be threaded, but they were not! I bought a tapping tool and put threads in the holes and now they are able to take the 8mm bolts. I had a problem with the "rivnut" installation, but I bought a pkg of them, along with a piece of sheet metal and I will do a bit of practicing first.All the factory bracket holes in the tub are already threaded except for the rear one on each side. A new set of Genuine Honda running boards comes with a Nutsert-style threaded insert for those two holes, which you can install using a hack bolt-and-nut-and-two-washers if you don't have the tool. Some have glued a nut onto a washer and slid it into the rail from a nearby larger hole. The bolt size is 8-1.25 IIRC, and you can probably substitute a 5/16" Nutsert in a pinch if you don't have access to the correct 8mm size.
Search the forum some and you'll find more on this. More than a few folks were blindsided by needing that insert put in. You could sometimes talk a dealer service guy into doing it while the car was in for something else. With the correct tool it takes less than a minute per insert to place and set them.
Now you worry me! There were no weld-in inserts. Just holes in the metal. The holes were thick enough to be tapped and threaded, but NO welded-in inserts.Wondering if yours were perhaps victims of undercoating or rustproofing? This is the first mention I've heard of the weld-in inserts not being threaded.