I was doing some work on my Motobecane, and needed a set of ball bearings for the steering head. Checked the old bearings with the metric caliper, and they read 3.98mm - I took that to be 4mm with a little wear.
The bearing supply house doesn't stock 4mm balls, so I went to the bike shop. They told me they were 5/32" bearings, and sold me 52 for $1.49. Went home and did the math. 5/32" = 3.97mm. How did I miss that?
Then I noticed that the hubs call for 5.5mm bearings. This time, 7/32" = 5.55mm.
Seems like these inch sizes interchange pretty well.