In a '90, if the inner locking pieces are still inside the brackets along
with stock knobs, there is only one reason they are holding: lubricate the
knob/bolt threads (in my best Scott Foss voice)! I have never had a problem
with '86-on tables staying securely in place. If that still isn't quite
enough, remove the locking pieces from the brackets and rough up the surface
with 40-grit sandpaper. That should give some bite.