NPT vs ANPT

THREAD DESIGN

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

The NPT and ANPT product screw threads in the sizes up to and including 2 inches are identical. Beginning at 2-1/2 inches NPT and ANPT product screw threads are NOT identical. Beginning at 2-1/2 inches the wrenching length of the coupling is different between NPT and ANPT product screw threads. NPT has 2 threads and ANPT has 3 threads. There are major differences in the gages.

PRODUCT ASSEMBLY

NPT and ANPT screw threads are designed to screw together. In most cases there will be no interference between the root and crest of the threaded at assembly. The tapered screw thread is designed to be assembled with some form of sealant to assure a leak free joint.

NPTF screw threads will screw together with both NPT and ANPT screw threads and should have no noticeable assembly problem. There will most likely be an interference fit between the root and crest on either the major or minor diameter of the screw thread, depending on which part is NPTF. To accomplish a seal on the joint, a sealant will be required.

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.

If your NPT product design requirements indicate additional measurements must be taken, consider changing the product requirement to ANPT per ANSI SAE AS71051. Because the majority of the taper pipe screw thread applications will be 2 inches or less; and because the NPT and ANPT product screw threads in the sizes up to and including 2 inches are identical; this should present no problems with the end product. If your product is over 2-1/2 inches some design consideration should be conducted.

ANPT requires: both L1 and L2 ring gages for the male thread; both L1 and L3 plug gages for the female thread; and Crest Check (frequently called 6-step) gages for both the internal or external thread.

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