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National Gas Taper vs. NPT

National Gas Taper vs. NPT

NGT = National Gas Taper per ANSI CGA V-1 Specification
NPT = National Pipe Taper per ANSI B1.20.1 Specification

Both NGT and NPT pipe screw thread forms have many similarities.

They have the same:
Lead
Pitch
Taper
Flank Angles
E0 Pitch Diameter
E1 Pitch Diameter
Major Diameter Truncation
Minor Diameter Truncation

The differences are:

1. The NPT screw thread connection is intended for transmission of non-hazardous substances, like water.

2. The NGT screw thread connection is intended for transmission of dangerous gases. Things that if leak cause death and destruction to the immediate area and maybe beyond. A mis-made NGT thread can be a product liability issue. If the thread that you made fails, you can be sure that the lawyers will be asking you to prove that you own the thread specification and that you used the fully battery of calibrated gauges on the parts as they were made.

3. The NGT is a longer screw thread than the NPT. The NGT minimum effective length is approximately 3 threads longer than the NPT.

4. The NGT screw thread has tighter control of the major and minor diameter truncations than the NPT.

5. The ANSI B1.20.1 Specification which defines the NPT screw thread only requires:
NPT-L1 Taper Screw Thread Ring Gage
NPT-L1 Taper Screw Thread Plug Gage

NOTE: The NPT-L1 Basic-Step taper screw thread gauge members are very similar to the NGT-L1 Basic-Step taper screw thread gage members, but not interchangeable. The pitch diameters are the same, but the minor and major diameters have tolerance differences in a direction that may allow a NPT-L1 Basic-Step taper screw thread gauge members to be modified into a NGT-L1 Basic-Step taper screw thread gage members. This is why the price of the L1 Basic-Step taper screw thread gauge member may be less than the other members.

6. The ANSI CGA V-1 Specification defines a complicated screw thread measurement system. There are rumors in the industry that the standard is being rewritten to simplify the screw thread gauge system. Many companies have been using the following tapered screw thread gages with full success:
NGT-L1 Taper Screw Thread Ring Gage
NGT-L8 Taper Screw Thread Ring Gage
NGT Crest-Check 6-Step Taper Plain Ring Gage
NGT Root-Check 6-Step Taper Screw Thread Ring Gage
NGT-L1 3-Step Screw Thread Plug Gage
NGT-L9 3-Step Screw Thread Plug Gage
NGT Crest-Check 6-Step Taper Plain Plug Gage
NGT Root-Check 6-Step Taper Screw Thread Plug Gage

This screw thread gauge system requires 3-step L1, L8 and L9 taper screw thread gage members, but most gauge makers only offer Basic-Step members. The 3-Step L1 and L9 plug gauge members are readily available for an extra cost. The 3-Step L1 and L8 taper screw thread ring gages require special gauge blanks and thus usually are available only with extended lead-time and increased price.

This system requires the Root-Check and Crest-Check taper gage members to be made as 6-Step, which is the form that the gauges are quoted by default.

NGT = National Gas Taper per ANSI CGA V-1 Specification
NPT = National Pipe Taper per ANSI B1.20.1 Specification
NGO = National Gas Outlet
ANSI CGA V-1 Specification = CSA B96
ANSI CGA V-1 Specification formerly ANSI B57.1

COMMENTS
The data provided is accurate to the best of my knowledge. Please use it at your own risk. Refer comments/corrections to gageguy@gagecrib.com.

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