A staybolt is a bolt or short rod commonly threaded throughout its length and used as a stay to connect opposite plates in a steam boiler that are subjected to a pressure tending to force them apart.
From the Smithsonian Collection:
Staybolt taps were used by boilermakers when installing or repairing the fireboxes of steam locomotives. Taps" are designed to cut threads into drilled holes, so that threaded bolts or threaded machine screws can be screwed in. The long tap was screwed into pre-drilled holes through the boiler's outer steel shell and into the steel firebox within the boiler. The tap cut threads into the hole in the outer boiler shell and also into a corresponding hole in the firebox. The tapped hole permitted a "staybolt" to be inserted and screwed into place; each staybolt held the boiler shell and the firebox firmly together."