Construction_and_Maintenance/Chapter_XI
The constant bending and re-straightening of the stays causes them to crack as at B and finally fracture as at C. When putting in new stays the holes are slightly enlarged, tapped for new screw threads, and fitted with correspondingly larger stays.
The presence of defective side stays is detected by tapping the heads with a light hammer, when an experienced man can tell them by the sound. Stay heads are generally burnt away by the flame.
Patch Screws in the Firebox.
In steam locomotive maintenance, patch screws (often technically called patch bolts) are the go-to solution for "blind" repairs. They are used primarily when a section of the firebox or boiler shell—typically in a "stayed" area—needs a patch, but the back side of the plate is inaccessible for the "bucking" required by traditional rivets.
Patch screws are employed when a section of the boiler plate has thinned due to corrosion or developed cracks. Because locomotive fireboxes are often surrounded by a narrow water space (the "mud ring" area), it is often impossible to get a person or a tool behind the plate to hold a bucking bar against a hot rivet.
Most patch bolts use a tapered thread (typically 3/4" or 12 threads per inch). This taper ensures that as the bolt is tightened, it creates a metal-to-metal wedge seal against the boiler plate.
The bolt usually features a square head used for driving it into the hole. In many designs, this head is necked down so it can be twisted off once a specific torque is reached, or it is simply cut off after installation.
Once the driving head is removed, the remaining portion is shaped to sit flush or slightly proud of the patch plate.
Patch screw information courtesy of Google Gemini search for "patch screws in steam locomotive firebox"