Originally Posted by
verktyg
Welding thin wall steel parts is tricky because it's easy to overheat the material and burn through the tubing. […] As @
dddd mentioned, it's easy for the lower meting temperature brass to contaminate the steel weld. My welding instructors would have beat me soundly about the head and shoulders with a bundle of welding rods if I ever did a weld like the one on this BB...
OTOH, this weld is on the thicker, bulge-formed bottom bracket shell. As others have mentioned, this is a fairly common failure point on such shells, usually a result of the seat tube not fully extending into the socket in the shell. This means the crack, and subsequent weld are below the level of brass penetration so contamination shouldn't be an issue.