got 15mm knuckles & want them to be 16mm?

[Serum]

Sounds like Robin made his holes with a reamer, if the guy did a good job by centering the two holes (aligning them parallel) it should be no problem (if the holes are a good tight/loose fit for the bearing). perhaps he told you he used a drill, but he used something different.

a drill has 2 cutting flanges, and an adjustable reamer has got many more. a drill has got the tendency to make triangle holes instead of perfect round ones.

I just checked a new racer-X I've got here; the front knuckles are 16.00 mm, and the rear knuckles are 15.98 mm

Typical size for a 16mm OD bearing is 15.97.