The hole is 0.41 inch, and can't be altered. I need to have a sliding fit on a rod, 18 inches in length, for this hole. Closest I can find is7/16 inch, or 0.4375 inch. 3/8 inch is too small.
Now the puzzle is how to reduce that diameter by methods which only available tools: a drill press, a hand drill, pedestal grinder, hand files, abrasive papers.
Obviously, I won't get much accuracy, but that's okay. One idea I had would be to chuck it into the drill press and push sideways with a file to the right diameter.
Hope that doesn't make you cringe. If it does, make a suggestion.