External bottom brackets have been appearing on higher-end bikes in the last few years. Can older bikes with bottom bracket shells that come stock with square taper internal ISO 68mm or oversized ISO 73mm bottom brackets be retrofitted with external cups when it comes time to replace the bottom bracket? I am well aware that the cranks will have have to be replaced, too.
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The external bottom bracket uses the same thread size and shell width as an older internal BB. They were specifically designed to allow use in a standardized frame without redesign.
They do require a new crank set, and depending on the model, and what is on your current bike, that may mean the purchase of extra components in order to be compatible with the correct number of gears/width of chain. If you are going from 9 speed MTB to 9 speed MTB components, for example, a like for like change, then it should be just the crank set and BB that change.
An external BB is also designed to fit on either a 68mm or 73mm shell. They are universal, as long as you dont have a pressfit BB, or an Italian thread BB. Similar crank sets are available for non-english thread BB's, but it must be designed for your frame's BB shell type specifically.
Yes they most definitely can be used on older frames - either 68 or 73 mm.
You may want to Google the version you are after e.g. MT51 on Shimano Tech Docs as they will have full instructions that you may want to read before you buy.