Screw Stays in Pressure Vessel

Basic description here:

http://www.farmcollector.com/steam-traction/steam-boilers-and-stay-bolts.aspx#axzz2l6FdG4mJ

Well, in the case of a firetube boiler, they are there to stop the inner firebox and the outer firebox wrapper from bulging apart from each other under pressure. Screwing them is the best way of getting them in. Once they are in, the outer is usually peened over like a rivet, and the inner usually has a sacrificial nut screwed onto it as it is easier to replace the nut when it gets burnt than the stay itself.

Have a Google at steam locomotive boiler construction and all manner of wondrous stuff will appear.