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So I've got a nice hybrid bike that I picked up secondhand, was doing some maintenance on it to get it up and running and thought I'd replace the bearings in the rear axle. In doing this, I've found a small pit in one of the cone nuts.

I don't have a picture, but this is the type of thing (In reality, mine's not as bad as this, it's less long and less deep):

enter image description here

Just thinking, would it be a really bad (or silly) idea to try to file the whole nut, so that the surface is smooth. I appreciate that this would be quite a lot of work to get right.

Cone nuts seem to vary so much and finding an exact replacement could be tricky, so thought I'd explore this idea...

Look forward to hearing your thoughts!

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    Yeah, take it to a bike shop (take the axle with, so the thread is a sure match). Most shops should have replacements. And be sure to replace all the balls while you're in there -- get those at the shop too. – Daniel R Hicks Apr 7 '15 at 20:47
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    Once you've gone through the hardened surface, your precision filing will have rendered the nut too soft to take the load. Moreover you'll almost certainly not achieve the right profile and/or roundness ... your bike shop will have a nut that fits even if not perfectly identical. – zeFrenchy Apr 8 '15 at 8:15
  • If you have access to a machine shop, it could be an excellent project from a machinists point of view. Has threads, curved surfaces, and hardening as components. In reality, it will cost more to make something than to buy a whole axle kit or a specific cone nut. Sometimes its about the learning and doing, not the cost though. – Criggie Nov 15 '18 at 6:14
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Unless you have a lathe the tolerances are going to be too high for you to be able to to do an acceptable job here. Plus, I believe that the cone is surface hardened, after you grind through that the underlying material will be too soft. If you visit your local bike shop they should be able to match it up.

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The cone you show is through-hardened, so it can be remachined to smooth and true; most cones are only case-hardened, so cannot.

I screw the cone right on to the axle, then put the protruding axle and into the chuck of a bench drill [so that the cone is close to the chuck jaws, and any bend on the axle will not affect the roundness], and use a diamond needle or warding file [bi-oval or round] to remachine the face by hand.

Don't get paranoid about the profile. As long as it approximately follows the original line, itwill work, and balls will find their own track.

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    OP said the cone nut pictured is an example, not the actual one. Out of curiosity, how would you know that a cone nut is through-hardened? – Argenti Apparatus Nov 14 '18 at 12:35
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    This might work in a pinch, but should only be done if the replacement part (which is generally quite cheap) is difficult to obtain for some reason. – Daniel R Hicks Nov 14 '18 at 12:51
  • I upvoted this back; I've done basically the same thing to fix cones except I use sandpaper or emery wrapped around a rod. It works, and there are a LOT of cones in the world you won't be able to find replacements for. I'm skeptical of the idea of being able to look at a cone and tell if it's case hardened or through hardened and also whether case hardening actually precludes this method from being usable. – Nathan Knutson Nov 14 '18 at 17:36
  • If it’s good enough for @Nathan... I’ll give it benefit of the doubt – Swifty Nov 14 '18 at 19:57
  • @NathanKnutson I guess, the damage pattern will be visibly different between a through hardened and a surface hardened nut. On a surface hardened nut, the initial damage should reveal the soft material beneath the hard shell, which will then take damage much more readily. This is just a wild guess, as I have zero experience with such nuts, but I think the nut in the picture might be a surface hardened one: You have this one, clear cut place where the damage starts, and after that, there seems to be nothing stopping further damage. – cmaster Nov 15 '18 at 0:07

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