Metric vs. inch ball bearings

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.

Metric vs. inch ball bearings

It depends on how tight your tolerances are, but for many applications you can swap 'em back and forth.

Re: Metric vs. inch ball bearings

Good point about the tolerances..

The wheel and steering bearings are adjustable, so I'd think that this very, very small difference wouldn't really matter.

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