The current design of the BBs one of the last holdovers from ancient
times that is still a perpetual problem. As I mentioned before, if it
has a left hand thread, the design is faulty, the thread being a crude
patch for the design problem, just as it is for pedal threads that
fail in spite of the orientation.

Left hand threads are used to cover for a moving interface that tends
to unscrews itself in use. As is visible on pedal crank faces, pedals
regardless of tightening, move in their contact with the crank and by
fretting erode the crank face. This erosion leads to cracks that
cause pedal eyes to fracture and throw the rider to the ground if
standing at the moment of failure.

Like pedals, BB cups also fret in the BB shell but, with the larger
thread, were moderately secure with right hand threads if tightened to
nearly unmanageable torque. That is why left hand threads are used
today to hide that they move. In time, the threads of the right hand
cup or BB shell become eroded to oblivion in time depending on which
of the two is the harder material. With steel hardened bearing cups,
the BB shell loses all its threads while with aluminum cups, the right
hand cup loses its threads.

Shimano has used a clamping method for left crank attachment to the
spindle that would also work for the BB. Meanwhile, cartridge
bearings fret on the inside and outside while the retaining cup frets
in the BB shell. That is what is wrong with the current design
especially with cartridge bearings.

In the short term, the current design works if monitored often and
maintained, but when the thread is gone, it presents a difficult
problem.

Jobst Brandt