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You're right sorry, spending more time reading energy efficiency stuff than bike things these days. External ones on a 68/73mm width will match up to internal on ~86mm if that's what it's expecting, externals on a wider frame might work for some big wide cranks with longer spindles but that'll be big tyred stuff.
snottyotter
Not all T47s are created equal. Like PF30, some use inboard bearings some use outboard. So if you’ve got an outboard T47 BB kicking about it’s not much use in a T47 wide frame, I believe from memory that TREKs actually use the widest of the lot.
Although basing a build around an old BB is quite mad