NPT, or National Pipe Thread, is a U.S. national technical standard for screw threads on threaded pipes and fittings. There are a tapered and straight thread series for different purposes such as rigidity, pressure sealing, or both.
Pipe Thread Types
There are many different specifications for pipe thread, the most common of which is NPT, which stands for National Pipe Taper, and this has a tapered thread for sealing and can provide a seal without thread sealant or teflon tape, though the usage of such sealants is sometimes recommended. Also common is NPS, or National Pipe Straight, and will only provide a seal with some kind of thread sealant. This thread is often used for rigidity purposes, and may be coupled with an o-ring. There are other pipe thread specifications, Such as NPSC, NPSH, NPTF, and NPTR, but they’re generally application-specific and don’t really apply to typical automotive uses.
Tapered Threads
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. The various types of pipe threads are designed for use both with or without thread sealants,[1] as particular applications demand. The sealants, where used, are typically thread seal tape or pipe dope (sometimes plumber’s putty). As the thread body is tapered (0.75 in/ft or 62.5 mm/m), a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricant like lubricating paste or thread seal tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
NPT Thread Specifications
| Pipe size | Thread density | Overall Length | Pipe Nominal OD | Tap drill | |
|---|---|---|---|---|---|
| 1/16 | 27 | 0.3896" | 0.313" | 15/64" | |
| 1/8 | 27 | 0.3924" | 0.405" | 21/64" | |
| 1/4 | 18 | 0.5946" | 0.54" | 7/16" | |
| 3/8 | 18 | 0.6006" | 0.675" | 37/64" | |
| 1/2 | 14 | 0.7815" | 0.84" | 23/32" | |
| 3/4 | 14 | 0.7935" | 1.05" | 59/64" | |
| 1 | 11-1/2 | 0.9845" | 1.315" | 1-5/32" |