National Pipe Taper (NPT) and National Pipe Taper Fuel (NPTF) are remarkably similar: both have the same taper and threads per inch, and the pitch diameter is the same. The difference is in the root and crest of the threads. NPT thread crest parameters encapsulate NPTF thread crests, even though the NPTF crests have a smaller range than the NPT crests. When screwed together, NPT thread roots allow clearance between the mating thread crests. To get a leak free seal between NPT pieces, a sealant such as tape or Pipe Dope, also called Ucky-Pucky, is required. NPTF threads are designed to deform during assembly, creating a mechanical seal between the mated roots and crests, without need for an extra sealant. NPT and NPTF can be connected without an adapter, but because of the NPT’s design a sealant will still be required.