Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.
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We focused on tools that would be useful to a commuter—someone trying to use a bike as a functional way to get around town, as opposed to riding recreationally (road biking and mountain biking). That said, it’s a wild and wooly bicycling world out there, and the streets are packed with so many different bikes, all shapes and sizes, new and old. Customizing your flat-fixing kit has advantages over buying a preassembled kit that always contains a tool (or two or three) that’s a piece of junk, or you don’t need. Or it’s missing something you do need. If you build your own kit, at least you know everything works. And you can add only what’s necessary for your specific bike without ending up with stuff you don’t need.