I recently hung some old doors that had been drilled to accept 1-3/4-in. locksets. These locksets needed replacing. To enlarge the old holes to house the new 2-1/8-in. cylinder sets, I had to make a jig that would center my hole saw over the existing hole.
I took a scrap piece of 3/4-in. stock, measured 2-3/8-in. from the edge to allow for the standard setback, and drilled a hole with my 2-1/8-in. hole saw. Then I clamped this guideboard over the existing hole, flush with the edge of the door, as shown. It acts as a guide for the outside of the hole saw itself, not for the arbor bit, and the holes come out clean and accurate.
Mark Messier, Eugene, OR
Edited and Illustrated by Charles Miller
From Fine Homebuilding #21