How many threads must be showing when bolting flanges?

When bolting flanges, it is essential for safety and performance that the threads of the bolt are completely through the nut. This ensures a secure connection and allows for the proper clamping force needed to seal the joint effectively. If the threads do not fully extend through the nut, there is a risk of inadequate engagement which can lead to failure, leakage, and potential safety hazards. Having the threads completely engaged ensures that the forces are evenly distributed and that the joint can withstand operational pressures. This practice is crucial in high-pressure piping systems where the integrity of the system is paramount.