I have a 2" boring head (Grizzly) for my x3 mill. It has a center hole and an offset hole to mount the boring bar vertically. It also has a horizontal hole. When I mount the boring bar in the horizontal hole I need to run the mill in reverse so the tip is cutting in the right direction. The problem I'm having is the head starts to unscrew from the collet. Is this not designed to use the bar horizontally? I have a 4" dia. hole in a 3/16" aluminum plate. I'm trying to countersink a 4 3/4" dia hole from the back. I made a few passes taking off 050" at a time. It was working, but now the head just starts to unscrew. Not having much experience doing this, I'm assuming I'm doing something wrong.

This should unscrew to change the arbor that the boring head mounts on. In your situation, you will need the opposite boring bars that you have. That is, if you don't want to tackle a simple modification.

You could drill out and tap a small hole on the outside edge or underside (drawbar side) of the arbor and boring head, then tap it for a small set screw. That will keep the indexing of the head and arbor together and should solve your problem without getting new bits.

Good luck on your project.