Identifying threads can sometimes be the most difficult and frustrating part of coupling selection. However, without the right combination of threads, you may not provide a functional or safe connection.

The diameters, threads per inch (TPI) and thread pitch, etc. are necessary to completely identify a thread. Ring, Plug and GO/NOGO gauges are required to accurately gauge or identify threads. In the field, in the absence of these gauges, thread leaf gauges can be used to identify the "Threads Per Inch" (TPI) and the thread pitch. On threads you have determined to be straight threads, a caliper can be used to measure the "Outside Diameter of the Male" (ODM) or the "Inside Diameter of the Female" (IDF). A caliper can also be used to take measurements of tapered thread diameters. However, these are more difficult to define because of the taper. Fortunately, there are few tapered threads to deal with and these can usually be identified from the nominal ODM and the TPI.

However, identifying the thread may not fully identify what is needed in a mating fitting. The application is the primary limiting factor on the thread type used. Dixon offers products with a wide variety of threads used with hose, pipe and hydraulics.

When attempting to choose a fitting, it is always advisable to first identify the thread to which it must connect. This may entail checking with a fitting or equipment manufacturer.

The fire hose thread specifications for some local municipal fire equipment and hydrants may vary according to local specifications. These can generally be most easily identified by contacting the local fire department responsible for the hydrant. The most common thread used on fire equipment is National Standard Thread (NST), also known as National Hose thread (NH).

When it is not possible to identify the thread:

  1. Determine the number of threads per inch by measuring the distance from peak of thread to peak of thread across the largest number of whole threads. Then divide the number of threads by the measurement (This will provide the TPI).
  2. Check to see if the thread is straight or tapered.
  • Straight Threads: Measure the "Outside Diameter of the Male" (ODM) or the "Inside Diameter of the Female" (IDF), from peak of thread to peak of thread.
  • Tapered Threads: Measure the "Outside Diameter of the Male" (ODM) at the large end and the small end, or the "Inside Diameter of the Female" (IDF) at the large end and the small end, from peak of thread to peak of thread. Then measure the Outside Diameter (OD) of the unthreaded pipe.

Once the application and these two pieces of information have been determined, the thread can generally be determined. When in doubt, contact the factory.

Straight Thread

Tapered Thread


When calipers and O.D. tapes are not available, the following method may be useful.

Cut a strip of paper not over 5/8" wide by approximately 18" long. Then wrap the strip tightly around the circumference of the male thread, making sure it is well back against the male shoulder, and the ends overlap. Then pierce once through both layers of the strip with a pin (Fig 1). After removing the strip, mark your pin holes by circling them as shown (or with arrows), to make sure the correct measurement can be taken. The distance between the pin holes, divided by 3.1416 equals the outside diameter of the thread.

Next, press your thumb firmly against the threads (Fig. 2) hard enough to make thread impressions on the strip (Fig. 3) thereby giving an impression from which the number of threads per inch can be obtained.

NYF – New York Fire Dept. Thread 1½" (2.100 ODM x 8 tpi) 2½" (3.030 ODM x 8 tpi)
CF - Chicago Fire Dept. Thread 1½" (1.933 ODM x 11.5 tpi) 2½" (2.990 ODM x 7.5 tpi)
NYC – New York Corporation Thread 1½" (2.093 ODM x 11 tpi) 2½" (3.000 ODM x 8 tpi)


If you are unable to determine the threads you need:

  1. Contact the Fire Department or Fire Marshall responsible for the area where the equipment will be located.
  2. Request the threads required for fire hose connections in that area consistent with your requirements. (usually 1½" and 2½")
  3. If this fails to get the information you need, contact our Fire Equipment Specialists at 877.712.6179 or 410-778-2008.

Always indicate the HOSE THREAD required when ordering!



Thread Compatibility

Abbreviation System Name Compatibility Seal Method
BSPP British Standard Pipe Parallel Male BSPP with Female BSPP
Female BSPP with Male BSPP
Female BSPP with Male BSPTr
BSPTr British Standard Pipe Taper Male BSPTr with Female BSPTr
Male BSPTr with Female BSPP
Female BSPTr with Male BSPTr
Female BSPTr not compatible with Male BSPP
CHT American Standard Fire Hose Thread (1" National Hose Thread is Chemical Hose Thread, also known as Booster Hose Thread) 1" Male NH (NST) with 1" Female NH (NST)
1"Female NH (NST) with 1" Male NH (NST)
1" Thread is used on both ¾" hose and 1" hose.
Not compatible with other systems
GHT Garden Hose Thread Male GHT with Female GHT
Female GHT with Male GHT
Thread is same for all size hose
Not compatible with other systems
IPS Iron Pipe Straight Thread Generic Name for Straight Pipe Thread
See NPSH for compatibility
IPT Iron Pipe Thread Generic Name for All Pipe Thread
More information required
NH or NST American Standard Fire Hose Coupling Thread (National Hose thread also known as National StandardThread) Male NH (NST) with Female NH (NST)
Female NH (NST) with Male NH (NST)
Not compatible with other systems
Thread pitch and diameters of fire threads may vary according to local and municipal regulations.
NPT American Standard Taper Pipe Thread
(National Pipe Tapered)
Male NPT with Female NPT
Male NPT with Female NPTF
Male NPT with Female NPSM
Male NPT with Female NPSH
Female NPT with Male NPT
Female NPT with Male NPTF
Female NPT not compatible with Male NPSM or Male NPSH
NPTF American Standard Taper Pipe Fuel Dryseal Thread (National Pipe Tapered Fine) Male NPTF with Female NPTF
Male NPTF with Female NPT
Male NPTF with Female NPSM
Male NPTF with Female NPSH
Female NPTF with Male NPTF
Female NPTF with Male NPT
Female NPTF with Male NPSM or NPSH
Note: NPTF with NPTF threads do not require sealant for the initial use. After that, sealant is required.
Not Compatible
NPSH American Standard Straight Pipe for Hose Couplings (National Pipe Straight Hose) Male NPSH with Female NPSH
Female NPSH with Male NPSH
Female NPSH with Male NPT
Female NPSH with Male NPTF
Female NPSH with Male NPSM
NPSM American Standard Straight Mechanical Joints (National Pipe Straight Mechanical) Male NPSM with Female NPSM
Male NPSM with Female NPSH
Female NPSM with Male NPSM
Female NPSM with Male NPT
Female NPSM with Male NPTF
Seal can be either mechanical or washer.
Mating fittings must be of same type.
SIPT Straight Iron Pipe Thread Generic name for Straight Pipe Thread Washer
TIPT Tapered Iron Pipe Thread Generic name for Tapered Pipe Thread Thread



Thread Dimensions

Nominal Dimensions of Standard Threads

ODM -- Outside Diameter of the Male   IDF -- Inside Diameter of the Female   TPI -- Threads per Inch

GHT (3/4") -- 1.0625 ODM, 11-1/2 TPI

NOTE: Female NPT (Tapered Pipe) thread is not available on hose swivel nuts.


Special Thread Information

2½" thread

threads per inch 6 7 8
outside diameter of male 3.058 3.125 2.990 (Chicago) 2.841 (NPSH)
3.062 (Pittsburgh)   3.052 (NST) 3.000 (NY Corp)
3.093   3.125 (Detroit) 3.3031 (NYFD)
3.125     3.062
3.156     3.076 (Cleveland)
3.187     3.093
3.234     3.140
3.250     3.156
3.312     3.312

Product Division

Dixon Powhatan