This is the 10mm mark:
I used a 3/8th socket to hold the plugs and grind them to a diameter of 10.1 mm.
Finished. Took some time but I had no choice (But they were only $.60 each).
Next to an original plug. Being deeper, there is more metal in contact. I used a small magnet to be sure that no metal was inside the journals. I have checked all bearings and no damage has been done, by the way.