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Oooops, overlooked this one. I didn't have to use non-standard bolts - the chainguard is really just another chainring with the teeth taken off, so as far as the crankset's concerned I'm running two chainrings so I could use the normal bolts.
I've never seen a Brompton with an external BB, although I can't think of any technical reason it couldn't be done provided you've got enough clearance between the crank and the rear triangle to permit The Fold to take place without contact.
Sorry for the dredge!
Upgrading/re-vamping a Brompton, had read this thread before and enjoyed it. Upon trying to do it myself I suddenly realised how many little issues you've overcome.
Mdcc mentioned a problem early on with the chainset using non standard bolts, was this overcome with the modification of the larger chainring?
I currently have a 24mm axle carbon chainset that I was looking to use but I'm getting mixed reviews on external bb's and their compatibility with bromptons - did you know if this is possible?
The other option I had was to get a campag double chainset but was unsure I'd be able to machine the same solution as you've been able to.