Editing Firebox (steam engine) (section)

===Sheets and stays ===
[[File:Firebox cutaway.jpg|thumb|Cutaway of locomotive firebox and boiler with radial stays]] 
The metal walls of the firebox are normally called ''sheets'', which are separated and supported by ''[[Boiler stay|stays]]''. The stays support and brace the "sheets" (plates) against pressure. Ideally, they should be located at right angles to the sheets, but since the outer sheet (wrapper sheet) is often rounded and the top of the firebox (crown sheet) is relatively flat by comparison, such a relationship to both sheets is impossible. The actual location of the stays is a compromise.  Since stay breakage is hidden, the stays have longitudinal holes, called ''tell-tales'', drilled in them which will blow water and steam, revealing if they are broken. A boiler with more than five broken stays, or two next to each other, must be taken out of service and the stays replaced. 

The [[fusible plug]]s, usually located in the highest part of the crown sheet, have a soft metal alloy core which melts out if the water level in the boiler gets too low. Steam and water blowing into the firebox both alerts the locomotive crew to the low water condition and helps put out the fire. Not all locomotives are equipped with fusible plugs.  Also, fusible plugs should be replaced at regular intervals, about every three months for a locomotive in regular service, because the soft metal alloy core will slowly melt out over time even if the boiler water is carried at proper levels. The "mudholes," or washout plugs, allow access to the interior of the boiler for washing and scraping away boiler mud and [[Fouling|scale]].

The sheets on the left and right are called "side sheets" while the sheet in the front of the firebox is the flue sheet. The "front flue sheet" is in the front of the boiler and at the rear of the smokebox. The "rear sheet" is at the back of the firebox and has the door opening in it. The ''crown sheet'' is at the top of the firebox. The crown sheet must be covered by water at all times. If the water level drops below the crown sheet, it will become overheated and start to melt and deform, usually sagging between the [[Boiler stay|crown stays]]. If the condition continues, the crown sheet will eventually be forced off the crown stays by the pressure in the boiler, resulting in a boiler explosion. This condition, usually caused by [[human error]] or inattention, is the single greatest cause of a locomotive boiler explosion.{{Citation needed|date=July 2021}}