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This is the original cone nut of a new coaster brake hub I bought recently. I didn't install it yet, it's new.

What bothers me are those weird stains / corrosion signs on the bearing surface. It looks like the chrome plating is damaged (or whatever plating is on them) by corrosion (caused by improper storage conditions?), or physical damage (over tightening?). Just my speculations.

The camera/contrast settings might make it look worse: enter image description here

Video (for different angles / light reflections):

Is it a good idea to just ride it and recheck its condition after a couple of rides? Or will it just fail right away? Can I polish it with sandpaper?

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    I will seek for a replacement, but in my town it is quite hard to find one. And I may have to wait for a week with an online order. – Robert Lee Jul 13 '18 at 0:09
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    It's hard to say without actually seeing it firsthand. It could be that what you see is basically "fingerprints" left over from whatever chemical was last used on the piece. Scrape your fingernail on the surface and if you don't detect any ridges that seem to correspond to the color changes then it's probably OK. – Daniel R Hicks Jul 13 '18 at 1:37
  • Pretty strange for a new part, that was presumably doused in grease. What made you take a new hub apart to find this? – Henry Crun Jul 13 '18 at 5:50
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    @HenryCrun I took it apart because I wanted to paint the shell and brake arm black (I found it easier to do that just by taking it apart and fully degrease it). And also because of my huge curiosity. The mechanism is amazingly interesting to me! I always wondered when I was younger how a coaster brake works. – Robert Lee Jul 13 '18 at 10:42
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    @RobertLee awesome work coming back to provide an update - that's much apprecialted. You should post that as an answer and accept it (using the check/tick) Stackexchange specifically allows this as a form of knowledge sharing. If you have a photo of the cone now, would be a fantastic addition to the answer. – Criggie Sep 12 at 9:38
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+100

Update: It's been over a year since I posted the question and I rode ~1000km on that bike. I had no problems with the hub. The cone looks better now, and performs well. I believe there were just some stains on the surface.

Here's a picture of it now, cleaned and degreased. I left the dust cap on now, because last time I took it off it was a pain to put it back on. I think the imprefections on the bearing surface are being accentuated a little by my camera's default edge enhancement algorithms; with the naked eye they're less visible.

cone

  • Points bonus for adding the extra info. Thank you. – Criggie Sep 29 at 7:39
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I'm just a guy who's serviced a few dozen cup and cone hubs in his day. I can't imagine that's going to last. Every cone I've ever had was highly polished, hardened steel. I've never seen plating. I've thrown out cones that were pitted less severely than the flaking on this looks, because they were pretty gravelly in use.

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I would try to remove it with very fine wet and dry (800-1200). (put cone in drill, sand off as it spins)

However I expect it is just a poor flashing of zinc or cadmium (both soft, shouldn't be cadmium nowadays), that would normally wear straight off the bearing surfaces as soon as ridden.

Aside from high-end cones, they don't actually come highly polished. Often just oil blackened, sometimes fairly coarsely ground, The balls polish them in service, and the oxide just wears off into the grease, and is presumably OK.

The important thing is that the steel is not pitted by corrosion.

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