stay-tap

from The Century Dictionary.

• noun A specially long tap, usually from 15 to 20 inches in length, though sometimes reaching 2 or 3 feet, used for tapping or threading the holes which receive the stay-bolts in the stayed surfaces of locomotive and marine boilers. The threads in both surfaces must be parts of the same screw in order that the threaded bolt may enter the thread in the second surface while fitting on the thread in the first. The lower end of the stay-tap is not screwed, but turned smooth in order to keep the tap truly concentric with the holes in both the inner and outer shells.