Just looking at the picture of the bolt I have to say I have only seen that style of bolt used to attach a “plate” that is no thicker than the depth of the countersunk flank. Is what you are calling a “block” actually a thin plate?
In other words, do those bolts pass through one part and get threaded into another?
Man, I wish I had the time to upload and post photos of the work area, so folks knew what I'm dealing with, but I just can't do it til this weekend.
The bolts I'm working on pass through an aluminum block/plate and then screw into a solid aluminum spacer rod, which has a mirror setup on the other side. Between the two blocks/plates, are the roller wheels, mounted to the shaft.
It's the separator rods that the bolts are actually tapped into, not the blocks/plates.