NPT threads need to be what to ensure a pressure-tight and water-tight seal?

NPT (National Pipe Taper) threads are designed specifically to create a pressure-tight and water-tight seal through their tapered shape. The taper allows for the threads to tighten together as they are screwed in, which helps to deform and compress the material of the pipe and fitting, forming an effective seal.

The taper in NPT threads means that as the threads engage in the fitting, they create a wedging effect that increases the contact area between them, enhancing the sealing capability and preventing leaks. This characteristic is crucial for applications where fluids or gases must be contained under pressure.

In contrast, options like flexible, reinforced, or flat do not adequately describe the key feature that allows NPT threads to fulfill their sealing function. A flexible thread would not maintain the necessary tightness under pressure, reinforced does not pertain specifically to the thread profile necessary for sealing, and flat threads would not create the necessary compressed joint. Therefore, the unique taper of NPT threads is essential for achieving the reliable seals required in a variety of mechanical and plumbing applications.