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I had to cut the crank arms off of a bike I'm working on. Used an angle grinder. Despite my efforts to avoid damaging the bottom bracket spindle, I nicked it in three places, all at the tip.

The bearings and surfaces are all dandy. Is it safe to use this with new crank arms?

Inline image This is the worst of the three.

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    You should carefully file down any raised spots around your nicks, so that the new arms seat tightly and don't get hung up on anything.
    – Daniel R Hicks
    Sep 21 '19 at 19:25
  • Curious - what was wrong that you had to cut the crank arms off but the BB was reusable? Guessing the Extractor threads were stripped?
    – Criggie ♦
    Sep 21 '19 at 22:53
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    @criggie yeah. I stripped them. I think a gorilla assembled this bike, and forgot the grease in a number of key areas
    – Fing Lixon
    Sep 21 '19 at 23:01
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The percentage of the total area of the tapers that is damaged is very small, so you should be able to re-use the bottom bracket. However, cartridge bottom brackets are quite cheap, so if the bearings are showing wear I'd just replace it.

IF you do re-use it, you'll need to carefully file down any burrs raised by the angle grinder, those will prevent a new crank seating on the taper properly.

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    Grease the spindle to avoid the same mishap next time you disassemble. And use a torque wrench if possible. Using cranks with built in extractor bolts is a good alternative to standard bolts. They are available as after-market upgrades, too.
    – Carel
    Sep 22 '19 at 17:12
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I'd reuse that spindle. The tips bear the smallest load. A nick near the center of the spindle would be more significant

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