Adjustable reamers are designed to ream odd sized holes. They are made of high-speed steel.

You can adjust them to any diameter you want, given the range of each reamer.

 Common rule of thumb in a machine shop is to drill ~.015" smaller than the reamer diameter measured at the center, or bulged part of the reamer. Obviously, it varies based on the size of the finished hole. Leaving only .002"-.004" in a hole for the reamer to remove does not work very well

To use an adjustable reamer, first pre-drill a hole slightly undersized. Next, adjust the reamer to the desired diameter, as measured at the center, or bulged part of the reamer using a micrometer, and then insert it into the hole. Turn the reamer clockwise with steady pressure, using cutting fluid for most metals (but not cast iron), until the hole is reamed to size.  

These are only meant to be used by hand, and only to shave off a thou or two at a time. Tighten up the adjustment until the center of the cutting edges starts to bind in the hole, then use a tap wrench to slowly work the reamer clockwise with suitable lubricant into the hole until it is through. The flutes bulge in the center, so that is the part that will do most of the cutting.

Never turn the reamer backwards to extract it; this will dull the edges. Either push it thru the hole or pull it out, always rotating it clockwise while you do. Remove the previously cut chips from the reamer, adjusting the screw a bit, and repeat as needed until you get the required hole.

WORK SLOWLY, IN SMALL STEPS FOR BEST RESULTS.