They are color-coded to the diameter. So, 1 1/2" is red,,apparently 1" is green, etc.
You can't countersink a bolt from the inside as the brackets won't sit flush w/ the tubing. You could shop off the head of the bolt (or use a headless bolt,,,) but you would render the bracket single-purpose by adhering the bolt to the threaded hole of the bracket. So there are several ways to mount the jack,,, I bolted the jack on from the outside with a 3" bolt and used two nuts (insert rim shot) sized slightly larger than the bolt as spacers,,,ie: so they slide on & off. Not quite as convenient as a wing nut when removing the jack for use, but simple & cheap. If this makes no sense let me know & I'll post a pic. :drink1:
Yes & yes.
I figure it will hold better than wing-nuts, although not quite as convenient.
I had wing nuts on my rack when I carried an ax/shovel bracket & had drilled holes in the wings & run bailing wire thru 'em because I didn't trust them to hold (unless they were rusted on).
This way I can grab a wrench & get 'em off fairly quick w/out blasting my fingertips on the wing-nuts,,, and I don't have to worry about the jack flying off at high-way speeds & coming thru my windsheild