Obviously, something needs to give and that something is the staybolt.
Larger locomotives made it evident that even the most flexible material would not hold up in the extreme distances from the mud ring, and the flexible staybolt was developed to help cut down on breakage.
The subject of the staybolt became irrelevant and was dropped.
Welded staybolts mitigated the hardness differential between stay and sheet material, but the change also deleted the requirement for a key feature that monitors the staybolt.
That change was a very bad idea for steam locomotive boilers simply because the feature, known as a telltale, is the best indicator of when the staybolt breaks.
A telltale is a 3/16-inch diameter hole on the longitudinal axis of the staybolt.
When a staybolt cracks, it typically cracks on one side only.
Square corners in fireboxes are easy to make but they impede circulation and often lead to a staybolt pitch problem on the wrapper sheets in the front and rear.
Staybolts were made from wrought iron or very ductile steel and they were invariably threaded through the sheets.