I'm building a tandem at the moment. The frame appears to be threaded for a 1.45" Chater Lea bottom bracket.
I'd like to fit a modern bottom bracket. Any ideas? Here's my ideas so far:
Threadless cartridge bottom bracket: Shell ID is probably way too big for these, but maybe make a sleeve to go inside? Will that give enough grip?
Machine a counter bore for press fit bearings: Doesn't leave a lot of material left on the BB.
Drill the ID of the cups to accept a JIS spindle: No idea how I'd calculate what size balls to use. Also cups are really hard so difficult to machine?
I should clarify, the question in hand is about the rear BB, which has a shell width of 74.5mm. Chain tension is not an issue, since the front BB is eccentric and I'm sure I've got a solution for that.