Difference Between ANPT and NPT Threads

So, what is the difference between ANPT and NPT threads? Unless you are an engineer, people can get confused about what those acronyms mean. In addition, most people don’t know the difference between the two types of threads. So, this will be an overview about what these threads are. 

The requirements for NPT (National Pipe Taper) screw threads are spelled out in ANSI B1.20.1
The requirements for ANPT (Aeronautical National Pipe Taper) screw threads are spelled out in ANSI SAE AS71051

Threads

Let’s start by clarifying what threading is. Just to make sure we are all on the same page. ANPT and NPT are both types of pipe threading. Threaded pipes are pipes that have a screw thread at the ends. The purpose is to help with assembly, as the added threading at the end allows effective connections. The types of threading at the end vary, depending on the purpose of the pipe. The history of pipe threading is rather interesting. However, it is time to talk about ANPT and NPT threads.  

ANPT and NPT Threads

First of all, let’s discuss the similarities between the two types of threading. Outside the fact they are threaded pipes, they are both regulated by the ASA (American Standards Standards). In addition, they are primarily used to transfer gas and liquid. However, there are distinct differences that need to be shown. 

NPT Threads

Let’s start off with NPT threads. NPT stands for National Pipe Threaded. These fittings are standard threading that is used in the United States. NPT consists of tapered threading which has ‘imperfect threads’ at the end of the threading. By connecting the male and female parts, the ‘imperfect thread’ makes a seal. Though torquing the threads, they compress against each other making an effective seal. Without this seal, leaks of vapor and liquid could occur. This could be dangerous if the content is hazardous. That is why NPT is prominently used in these situations, as it can make a flawless piping system. However, NPT threads are not recommended for disassembling and then resemble. That is because it weakens the sealing abilities of the pipes. 

This is one of the most common types of threading seen in pipes. 

ANPT Threads

ANPT threads are both similar and different from its counterpart. Internally, ANPT is the same as NPT. However, the applications and other factors are what make these pipes so distinctive.  ANPT is an acronym for Aeronautical National Pipe Taper. These pipes have been used prominently by the Navy and Army. In order to use these pipes in aeronautical sphere, the pipe threading is carefully controlled in regards to diameter, taper, and thread form. This strict regulation on these threads allows them to be carefully crafted to have an effective seal that handles the stress of aeronautical uses. 

As you can see, NPT and ANPT have differences that make them useful in different industries.

GAGE REQUIREMENTS

Both NPT and ANPT screw threads have a requirement for an L1 gage. The gage is called L1 because it relates to the L1 dimension in the screw thread specification. Even though the L1 requirement is identical for both NPT and ANPT product screw threads, the tapered pipe screw thread gages are different at either the major diameter or the minor diameter. From my quick survey of the two standards I found: the ANPT-L1 tapered pipe screw thread plug gage major diameter was larger than the NPT major diameter; the ANPT-L1 tapered pipe screw thread ring gage minor diameter was larger than the NPT minor diameter. All other L1 gage parameters are identical between the NPT-L1 and ANPT-L1 gages. Technically; the NPT-L1 and the ANPT-L1 gages are not fully interchangeable.

The NPT screw thread standard only defines a L1 ring gage for the male thread and a L1 plug gage for the female thread. Both gages measure the hand tight L1 length of the screw thread.

There are no such NPT tapered pipe screw thread gages: L2; L3 or 6-Step Crest Check. If you have NPT tapered pipe screw thread gages marked: L2; L3 or 6-Step Crest Check, they are really ANPT gages which have been inappropriately remarked to meet a customer's demand. The gage maker should have educated the customer instead of supplying the incorrectly marked taper pipe screw thread gages. If these inappropriately marked taper pipe screw thread gages were sent to a calibration laboratory, the calibration laboratory should return the gages without calibration because there is no standard which defines the gages.