Replacing the one outer will be OK, but it you want maximum longevity then it makes sense to do all three.

Although it's not happened to me, that's not the first bearing cage I have seen break up. You just don't know if one of the others is going to go too.

Regardless of what level of work you have done, I would personally never take my crank to anyone that doesn't have (a lot of) previous experience.
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Andy.
NSR-WORLD.COM

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