Now that the firebox sheets have been replaced, the primary focus is now on the installation of the staybolts. Approximately 300 staybolts were cut out in the areas where the new firebox sheets were applied.
The firebox sheets are supported by many stay bolts. The boiler shell (wrapper sheet) around the firebox has a water space between the inner and outer sheets. The staybolts function is to hold the boiler shell and the firebox firmly together.
Diagram of a Flexible Stay-bolt showing the wrapper sheet (left) and the firebox sheet.
The original stay-bolts were 3/4” diameter according to the original build sheet. The new stay-bolts are 1” in diameter. They are increased in size due to repeated renewal of the threads during the tapping process. Re-threading the staybolt holes ensures a snug fit with the new staybolts. The holes are stepped up in 1/16″ increments.
A staybolt tap is used to clean up the threads prior to installation of the stay bolts. An air motor is used to turn the stay bolt tap to cut threads in the wrapper and firebox sheets. The staybolt is then threaded into the sheets and the ends peened over with an air hammer.
While the staybolt is being driven with a pneumatic hammer it is simultaneously being braced. To prevent damaging the staybolt’s threads, the opposite end must be supported. A bucking bar is placed on the end of the staybolt to back it up.