1 3/8" is a pretty big hole. I'd suggest scribing a line the diameter of the hole you want right down to bare metal (maybe make a template to trace around). Then center punch and drill a series of 1/8" holes all around the perimeter and "connect the dots" with a small saw blade, or a 1 1/4" hole saw, etc. Finish off the hole with a half-round file. A hole saw will wander unless the drilling is done in a drill press with the workpiece clamped firmly to the press table.
Jerry


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