Getting ready to put my 250+ pound bumper back on. The flimsy frame mounts are long gone so this is my simplified cure. The Large bolt is a M18 Grade 8.8. The diameter snugs into my frame rail's hole perfectly on the outside of the frame rail. On the inside there is a square hole but having used this setup on one side for a long time, it didn't move a smidge since it is pinned on the outside anyway. The 10.9 bolts replicate the stock bolts that used to hold the bumper on. This will pull the bumper squarely up against the front of the frame rails. I know it sounds too simple but if it works as good as just the outer one that I have used for years, I think I will be okay.
This is what the other one looked like (outboard only - I didn't make an inboard until tonight).
Now all I need to do is to
x-ray them, penetrant, glass bead, polish, zinc plate, powder coat and plasti-dip and they'll be ready to hold a bumper tomorrow.