I've had to remove the rear BB from our Bertin tandem, so it can have some beefier dropouts, plus gear hanger, brazed on at Argos.
This gives me an opportunity to look at the existing BB and consider options. It's a traditional TA ball-bearing and adjustable cups jobby, and the spindle has TA 374 stamped on it. I've been able to remove the adjustable cup, and its diameter appears to be 35mm. This, together with web info on the 374 stamp, tells me it's a French threaded BB. This would make sense on a French tandem, although the pedal threadings are standard, as on any British bike.
Consulting
Sheldon, he lists, under TA:
#374 123.5 mm (road triple)
but my 374 spindle differs; it's 121.75mm, not 123.5mm.
The A, B & C sections measure 39.0mm, 56.0mm & 26.5mm.
I can't get the fixed cup out at the moment, and I don't want to get heavy with it in case I'm wrong about the threading!) but, encouraged by the reasonable state of the existing cups and spindle (despite a broken ball bearing on one side!), I'm going to reassemble it with news balls for the time being, while I consider upgrade options.
I'm also upgrading from a 6-speed freewheel to an 8-speed cassette, so the chainline on the right side could reasonable move outboard to centre on the cassette, but I'd need to keep the chainline on the left to line up the timing chain, or change both BBs to ensure the same timing chainline front and back.
What does the panel see as my options? Has anyone upgraded in a similar way? Would a UN73 or similar be OK with TA cranks, if I could get the French threading? Would such a replacement be symmetrical, unlike the existing TA one?
