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I have a rescued bike with rear Maillard Helico Matic hub. I decided to replace the ball bearings and removed the old ones. Cages, balls, and cones looked ok. I measured old balls and they were cca. 3,7mm in diameter. I looked around on the intewerbs and found this site with the picture below. The hub on the picture looks exactly like mine. Well, the decal has a bit different graphics but by eyeballing everything else is identical. The picture says that this hub takes 5/32" ball bearings, which is bigger than my measured 3,7 mm by about 0,2 mm. Is it possible for ball bearings to wear this much?

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    It's unlikely that ball bearings wear this much. 5/32'' is 3.97mm - so it would be even more than 0.2mm. How did you measure the diameter? Could you post a picture of the measurement?
    – Christian Lindig
    Nov 6, 2017 at 18:18
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    @ChristianLindig I have used Vernier caliper. I know how to use it. And... no pictures taken. I find it also odd for ball bearing to wear this much and still remain round. I have measured balls in different orientations. I suspect that someone refilled the bike with wrong size ball bearings.
    – o115208
    Nov 6, 2017 at 18:23
  • Unless subjected to corrosion or excessive dirt ball bearings do tend to remain remarkably round as they wear. 0.2mm sounds like quite a bit, but not beyond the realm of possibility.
    – Daniel R Hicks
    Nov 6, 2017 at 22:36
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    You're assuming they started at the nominal size. I suspect someone's replaced them with a set of slightly smaller ones, which is fine if they're all the same size. You'd be better getting the correct size for a rebuild though - bearings are cheap.
    – Criggie ♦
    Nov 7, 2017 at 1:30
  • What is the source of the photo? I find it odd that the cones and locknuts are metric while the bearings are imperial.
    – JohnP
    Dec 15, 2017 at 15:38

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When bearings wear down, they tend to wear unevenly. They pit and chip and generally come out of round. The races tend to look equally rough if they're the same age as the bearings. If all is looking to be in fairly good shape, I think it's far more likely that someone has previously serviced the hub and used slightly smaller bearings.

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    When bearings pit and chip and go out of round they wear more rapidly, but bearings can wear substantially with no visible problems. This is why if you replace one ball you should replace them all, since a new ball is apt to be larger than the rest, and that will cause the bearing cups/cone to pit and wear out of round.
    – Daniel R Hicks
    Jan 21, 2018 at 2:09

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