Thread Identification & Types Guide

Knowing the correct thread is critical for selecting the right part. Selecting the wrong part can result in damage to the thread during installation, which compromises the pressure holding capacity and seal reliability of the fitting or adapter. Quickly and properly identifying threads can help maintain safe, productive and profitable operations.

Thread Types

Steps to Thread Identification

Step 1 – Determine if the thread is tapered or parallel

In some cases, Step 1 can be accomplished by visual inspection alone. Tapered threads get smaller in diameter toward the end of the fitting while parallel threads maintain the same diameter from start to finish. If this is not obvious by looking at the fitting, use the parallel jaws of a caliper to make a comparison. Furthermore, the presence of an O-ring or the removal of a tube nut is usually an indication that the male thread is parallel.

Step 2 – Determine the pitch

This can be deciphered using a pitch gage for comparison or by accurately measuring and calculating the number of threads within a given distance. It is much easier to compare threads against a lighted background with a pitch gage. Because some thread pitches are relatively similar, it is advisable to try a number of gages before deciding which one fits best. The result from Step 2 will narrow down the possible thread forms even more because most have a distinct pitch.

Step 3 – Determine the size

Combining the results of Steps 1 and 2 will determine – or help predict, in some cases – the correct procedure for Step 3. There are two methods for determining the thread size – which to use depends on whether the thread is a pipe thread or is not a pipe thread. Keep in mind that tapered (as determined in Step 1) does not necessarily mean that it is a pipe thread (e.g., Metric Tapered). Likewise, pipe thread can be parallel (e.g., BSPP). For pipe thread, determine the size by comparing it with a nominal size profile, as shown in Figure 1 (a useful tip – pipe sizes up to 2” nominal size can be determined by measuring the actual outside diameter, subtracting ¼” then rounding-off). For non-pipe thread, the actual size can be determined by measuring the outside diameter (major diameter) with a caliper, as shown in Figure 2.

Step 4 – Designate the thread

Technically, this final step does not pertain to identifying the thread. Rather, it is a method of designating the thread type in an industry standard format for others to understand. These typically have an indication of the thread size (whether nominal or actual), the type and – in some cases – the pitch.

The tables and images below and assist you in identifying the right thread type to suit your needs.

Thread Type Detail

American Connections

Four-Bolt Flange (SAE J518 and ISO 6162)

The Four-Bolt Flange is suitable for connecting 1/2" to 3" hoses and tubing and it is widely used in fluid power systems. The O-ring seats on the ring groove of the male. The seal takes place between the O-ring on the male and the smooth face of the female port. Four bolts and two clamp halves hold the connection in place.

SAE J518 flanges are available in two pressure categories: standard (code 61) and high pressure (code 62).

Flareless Compression (SAE J514)

The male of the flareless tube has a 240 seat and a straight thread. The female half has a compression sleeve and a straight thread. In addition to the sleeve, a female nut and a tube form part of the female connection. On the male, the seal takes place between the 240 seat and the compression sleeve. On the female, the seal takes place between the tubing and the compression sleeve. The threads engage to form a mechanically strong connection.

JIC 37° Flare (SAE J514)

This connection finds many applications in hydraulic systems. Both the JIC male and JIC female have a 37º flare seat and straight threads. The male and female flare seats seal when the straight threads are engaged. The connection is held mechanically by the straight threads of the male and female halves. It is important to note that most SAE J514 threads are identical to SAE 45º flare threads but their seating angles are different.

NPSM

The male and female threads of the NSPM connection are straight. In addition, the male has a 300 internal chamfer while the female has an inverted 300 seat. A mechanical connection is formed when the two halves are threaded together. The tapered seat forms a leak-resistant joint. NSPM connections find many applications in fluid power systems.

NPTF

The NPTF is a dryseal thread. The male thread mates with the female thread and a seal is formed when the two are crushed together i.e. threads deformation. Teflon and pipe dope are commonly used when additional sealing is needed. Although this connection is widely used in fluid piping systems, the National Fluid Power Association (NFPA) does not recommend it for hydraulic applications.

O-Ring Face Seal (SAE J1453)

This O-Ring Face Seal connection offers an impressive leak resistance and it is suitable for applications that are up to 6000 psi. The male contact has an O-ring and a straight thread. The female contact has a flat surface and a straight thread. A seal is formed when the O-ring in the face of the male end is compressed onto the machined flat surface female seat. The swivel female nut mechanically holds the connection.

O-Ring Pilot Threads

The O-Ring Pilot Threads connection is commonly used in vehicle and commercial air conditioning systems. The male and female ends have a pilot and a seal is formed when the O- ring is compressed. The pilot can either be long or short. Threads engage to form a mechanically strong connection.

SAE 45° Flare (SAE J512)

The SAE 45° Flare connections are commonly used for low pressure applications such as refrigerant lines, fuel lines, and automotive piping systems. Both the SAE male and female halves have a 45° flare seat. The threads of the two halves engage to form a strong mechanical connection. The seal is formed on the 45° flare seat. Except for the seat angles, the SAE 45° Flare connectors are identical to JIC 37° Flare connectors.

SAE Inverted Flare (SAE J512)

The inverted flare finds many applications in automotive systems. A machined male connector has a 420 seat.A flared male tubing has a 450 seat. The female end of the inverted flare has a 420 seat that provides a sealing surface. Threads engage to form a mechanically strong connection.

SAE Straight Thread O-ring (O-Ring Boss)

SAE J1926-1 and ISO 11296-1

The male connector of the O-Ring Boss has an O-ring and a straight thread. The female port has a sealing face, a chamfer, and a straight thread. The O-ring is compressed into the chamfer to form a seal. The threads of the two halves engage to form a mechanically strong connection. This connection finds many applications in high pressure hydraulic systems.

International

BSPP

The BSPP male end has a 300 seat. The tapered nose of the female swivel seals on the 300 seat of the male. Although the male end is similar to the American National Pipe Straight Mechanical (NPSM) male, the two are not interchangeable because their thread pitches are different.

BSPT

The tapered male of the BSPT mates with a tapered female. The seal takes place on the threads. Although the BSPT male end is similar to the National Pipe Tapered Fuel (NPTF), the two are not interchangeable because their thread form and sizes are different.

Flat Face Port with BSPP Threads (ISO 1179-1)

DIN 3852

The parallel threads form a seal using a variety of sealing rings or washers. The seal takes place between the male end and the machined flat surface of the female.

ISO 261 Metric threads

ISO 6149 Metric Port and Stud Ends

ISO 261 threads & O-ring seal

The male connector of the ISO 6149 connection has an O-ring and a straight thread. The female half has a machined surface, a chamfer, and a straight thread. The O-ring on the male sits on the chamfer of the female port. The ISO 6149 connection is similar to the SAE J1926-1 O-ring Boss except that the former has metric threads. The O-ring on the male connector is compressed into the chamfer to create a seal. The straight threads engage to form a mechanically strong connection.

JIS 300 Flare

The male has a 30° seat and a straight thread. The female half has a 300 seat and a straight thread. This connection is similar to 37° Flare. Its 30° seat and BSPP-like thread dimensions differentiate it from the American 37° Flare.

Komatsu 30° Flare (JIS Metric)

The Komatsu 30° Flare has parallel metric threads and a 30° seat. This connection is widely used on Komatsu equipment. The JIS metric connection is similar to the JIS 300 flare, except that the latter has BSPP-like thread dimensions.

Metric 60° Cone

DIN 7631

The male of the Metric 60° Cone connection has a 60° recessed cone and a straight thread. The female half has a globeseal seat and a straight thread. The seal takes place between the 60° recessed cone and the tapered nose of the female. The threads of the two halves engage to form a mechanically strong connection. This connection finds many applications in hydraulic systems.

Metric Tube Compression (DIN 2353 24° Cone)

The male has a 240 Cone and a straight thread. The three female connectors have straight threads and a sealing surface. The seal takes place between the 240 Cone on the male connector and the sealing areas on the females.

Fittings are available in two series: the DIN 2353 L (light) and the DIN 2353 S (heavy) classes. Each series has its own tube sizes and thread dimensions as shown in the table below: