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I bought a used 1990's steel bike the other day and have disassembled it for repainting. However I came across a small patch of missing paint on the drive-side chainstay, which had apparently been removed to check up on a small hairline crack. It doesn't seem like an immediate safety hazard, but I'm concerned it will continue to grow.

Having the frame professionally repaired would probably be prohibitively costly, so what are the DIY alternatives? The best solutions I could come up with is to either use epoxy glue and steel strips to reinforce the area (but epoxy is apparently not ideal for some reason), or drill two small holes at the end of the stress crack to prevent further propagation, then fill in the holes somehow. I do have a soldering iron, which I'm pretty sure isn't a recommended solution.

Any advice is appreciated. Thanks.

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    Are you talking about the vertical line? It's probably a scratch. At that spot on a steel frame there's no stress to form cracks.
    – Daniel R Hicks
    Aug 14, 2021 at 0:51
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    Oh, if so then that's great news. The surface is slightly indented, so I initially assumed that the damage was caused by physical impact, ie from bumping into walls. But then I realize that the area is behind the crankset so accidental bumps shouldn't be possible.
    – Morgan087
    Aug 14, 2021 at 1:35
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    Is there a small hole near the chainstay (weld gas escape hole)? If so, squirt a little penetrating oil into the chainstay and see if any makes its way out of the “crack”. Adds a little rust resistance too :) Alternatively, get some fluorescent liquid (ie highlighter juice) and use UV light to spot seepage.
    – MaplePanda
    Aug 14, 2021 at 3:00
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    At least from the photo it’s hard to tell how deep this scratch is. I’d try to sand it or file it down to get an idea. Could just be from the chain trapped between chainstay and chainring.
    – Michael
    Aug 14, 2021 at 5:40
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    It's definitely a spot where you'd expect vertical scratches.
    – Daniel R Hicks
    Aug 14, 2021 at 12:47

2 Answers 2

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As others have pointed out, this mark is more likely to be the result of the chain slipping and getting caught between the chainring and chain stay, than a stress crack caused by regular use. It may still be a stress riser and it's probably best to reinforce the area somehow. I'll look into drilling stress termination holes at the ends of the mark and getting them professionally filled in with brazing material, depending on the cost.

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What I see may or may not be structural, it's hard to tell. First take a flashlight and try to estimate its depth, possibly spreading the chainstay around. If it seems to not be very deep, I'd take a Dremel-like tool and remove a very small amount of material from the surface. The most important thing with a Dremel is that if the crack goes on and on, and doesn't vanish, stop! It's better to have lots of material and a larger crack for welding than it's to have little material and a smaller crack. Bicycle tubes are very thin walled so they don't have lots of unnecessary material.

Even a surprisingly small scratch can actually act as a stress riser so it's in your interests to remove it by making it smooth.

If you can't remove a minimal amount of material and thus eliminate the scratch, and instead the scratch seems to be very deep, take the bike to a professional welder if you consider the frame valuable enough for repair (for example if it's chromium molybdenum steel and butted and you consider the brake attachment the kind of attachment you want it's probably valuable enough even though the headset might not be the standard used today, i.e. 1+1/8" threadless). It will cost a lot but still the cost could be less than what an equivalent frame costs today. Remember that you have to pay over $500 today to get anywhere near decent steel frame. You can easen the job of the welder by removing paint from a large area around the crack and then telling that you'll do the repainting yourself. If you do the paint removal and repainting yourself, I can't imagine how a welder could possibly spend more than an hour on the small crack.

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  • Professional welding is out of the question. It's a nice butted cr-mo frame but that's not enough to justify the cost for me (welding costs more than another cr-mo frame on the used market), not to mention that welding weakens steel. Also I don't trust my skill and experience to be able to tell when to stop if I were to level the area with a dremel. So I still like the gist of my second idea, though I don't know how viable it is.
    – Morgan087
    Aug 14, 2021 at 17:02

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