Tom Sherman wrote:

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.

I just throw out the bicycle if I have problems with the bottom
bracket.

In contrast, I had my involuntarily threadless BB bored out and a
hardened threaded steel sleeve silver soldered to keep my custom frame
of many years in service. I have a great frame builder and bikie
friend who has great metal working skills who also made the pedal
crank modification which solved the pedal to crank problem.

Jobst Brandt