[quote=Dogboy]If you have access to thru-bolt with nuts on the inside of the crossbar, that would be ideal. Even if it is a long reach, you may be able to attach an extension to your wrench (use a piece of pipe or similar) to hold the nut inside the beam.
I agree, a barberhauler can have quite a lot of load. Probably roughly the the same as your jib sheet.
Another option would be to take a separate aluminum plate which is thicker than the crossbar (say 3/16" - 1/4" thick) so you can get plenty of thread engagement. Drill and tap the plate and screw the cleat onto the plate. Then use rivets to attach the cleat/plate assembly to your crossbar.
Using rivnuts would also be a good option, but a special tool is required to set them.
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