Originally Posted by cburger
It is generally possible to get a hole saw to match the transducer size and just enlarge the existing hole to fit the transducer you wish to install.
When enlarging an existing hole, with a Hole Saw, you need to provide a temporary plug
for the hole saw’s Pilot Drill (Arbour). Clamping a 1/4" piece of plywood
over the existing hole is one easy solution to the problem of hole saw “wandering”.
Another technique is to install two hole saws on a single
arbor, one inside the other. The larger, outer saw should be the size of the new hole, and the inner saw should be sized to the existing hole and protrude at least a half inch beyond the outer saw.
Chuck the arbor into a drill and insert the smaller saw into the old hole. Then squeeze the drill trigger and carefully guide the saw through the hole to keep the drill on a straight, steady course as the larger saw cuts a new hole around it.