When I spent some time in a repair shop, never quite knew that one of our line boring bars would suit a particular job, when a pump came in. Small shop-made bars haven't room for an adjustment mechanism as MachineNZ points out.
So, a creative member made a clamp on adjuster. It was a Vee-block with a 10-32 screw penetrating through the vee's center, making the indicator perform reliably.