Two things, first: bushings. You can open up the bore in a gear and install a bronze bushing. You can size the bore to accept a stock bushing. Second : don't fear the 4 jaw. You can center it up in two turns. Chuck it up close to center and set up your indicator. turn one full revolution noting the high and low numbers. Split the difference and you have your magic number. With your magic number line up a set of jaws with your indicator and adjust them to that number, turn 90* and repeat. Check it out, you should be done. IE: my numbers are .010 low and .040 high. A difference of .030. Half of that is .015. I add .015 to .010 and get .025. .025 is my number. I dial both sets of jaws to this number and check. I should be really close. Good enough if measuring thousandths not tenths. Alternately, when I achieve my magic number I can rotate the chuck until it reads that number and then ste the dial to 0. Now I set the jaws to 0. I just don't take that extra step myself. Two chuck keys help speed things along and would be a good learning project with your milling attachment.