Countersinking a hole is to cut a conical hole into a metal object. This process is normally used to create space for the head of a screw or bolt to sit flush with the surface of the material or to clean away burrs left from drilling and tapping.
Countersinking drilled and tapped holes on piping is good practice to prevent corrosion and to create a tight seal when screwing in the relevant fitting during a trenchless installation project.
Trenchlesspedia Explains Countersinking
A countersinking cutter bit is shaped to widen a drilled hole into a conical shape. Tiny fragments of metal left attached to the edge of a drilled and tapped hole can get caught in the threads of the fitting. These metal fragments will damage the threads, resulting in a loose fit. It will also affect the seal of the parts together by creating paths for leakage.
In trenchless projects, where piping is installed underground, the quality of workmanship is very important to prevent problems once the installation is complete.