14.0 Threads Used For Tube Fittings

Different types of screw threads have evolved for fastening, and hydraulic systems. Of special concern are plastic-to-metal, taper/parallel threaded joints in hydraulic circuits. A discussion and recommendations are provided to create an awareness of different types of threads and how they are used.

14.1 Evolution of threads
In the nineteenth century, many different types of screw threads were required for hydraulic and pneumatic circuits as well as fastening components. As a result, manufacturers started to devise their own fastening systems. This resulted in compatibility problems. The English mechanical engineer and inventor, Sir Joseph Whitworth devised a uniform threading system in 1841 to address the incompatibility problem. The Whitworth thread form is based on a 55 degree thread angle with rounded roots and crests.

In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered (NPT) Thread in 1864. His 60 degree thread angle, in common use by early American clockmakers, enabled the American Industrial Revolution. These thread forms later became the American National Standard.

The Whitworth thread form was selected as a connecting thread for pipes, which was made self sealing by cutting at least one of the threads on a taper. This became known as the British Standard Pipe thread (BSP Taper or BSP Parallel thread). The Whitworth thread is now used internationally as a standard thread for jointing low carbon steel pipes.

The best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal is the American National Pipe Tapered Thread, or NPT Thread. NPT has a tapered male and female thread which seals with Teflon tape or jointing compound.

14.2 Type of Threads
Pipe threads used in hydraulic circuits can be divided into two types:

I. Jointing threads – are pipe threads for joints made pressure tight by sealing on the threads and are taper external and parallel or taper internal threads. The sealing effect is improved by using a jointing compound.

II. Fastening threads – are pipe threads where pressure tight joints are not made on the threads. Both threads are parallel and sealing is affected by compression of a soft material onto the external thread, or a flat gasket.

14.3 Sizes
Pipe thread sizes are based on an inside diameter (ID) or flow size. For example, “1/2 –14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14 threads to the inch, made according to the NPT standard. If “LH” is added, the pipe has a left hand thread. The most common global pipe thread forms are:

NPT - American Standard Pipe Taper Thread
NPSC - American Standard Straight Coupling Pipe Thread
NPTR - American Standard Taper Railing Pipe Thread
NPSM - American Standard Straight Mechanical Pipe Thread
NPSL - American Standard Straight Locknut Pipe Thread
NPTF - American Standard Pipe Thread Tapered (Dryseal)
BSPP - British Standard Pipe Thread Parallel
BSPT - British Standard Pipe Thread Tapered

Plastic injection molded thread forms are manufactured to ANSI B2.1 and SAE J476 standards. The word “tapered” in several of the above names points to the big difference between many pipe threads and those on bolts and screws. Many pipe threads must make not only a mechanical joint but also a leak proof hydraulic seal. This is accomplished by the tapered thread form of the male matching the thread form of the female tapered thread and the use of pipe sealant to fill any voids between the two threads which could cause a spiral leak. The bottoms of the threads aren't on a cylinder, but a cone; they taper. The taper is 1⁄16 inch in an inch, which is the same as 3/4 inch in a foot.

Because of the taper, a pipe thread can only screw into a fitting a certain distance before it jams. The standard specifies this distance as the length of hand tight engagement, the distance the pipe thread can be screwed in by hand. It also specifies another distance – the effective thread, this is the length of the thread which makes the seal on a conventional machined pipe thread. For workers, instead of these distances, it is more convenient to know how many turns to make by hand and how many with a wrench. A simple rule of thumb for installing tapered pipe threads, both metal and plastic, is finger tight plus one to two turns with a wrench. Torque installation values can be determined as per application, but due to the variations involved in pipe joints such as dissimilar materials of male and female threads, type of sealants used, and  internal variations in product wall thickness, a standard torque specification cannot be generically applied.

This table shows the distances and number of turns called for in the standard. A tolerance of plus or minus one turn is allowed, and in pract ice threads are often routinely cut shorter than the standard specifies. All dimensions are in inches.

Table-XVI-American Standard Taper Pipe External Thread

14.4 Taper/Parallel Threaded Joints
Despite the standards created to maintain uniform fittings, tapered pipe threads are inexact and during the course of use and repair the threads can become damaged and susceptible to leakage. The area where the crest and the root of the thread meet can form a spiral leak path no amount of tightening will eliminate.

A pressure tight joint is achieved by the compression in the threads resulting from tightening. This compression and sealing occurs in the first few turns of the internal thread. As wrenching takes place, material from both the male and female threads deform into each other. This ensures full thread contact which minimizes spiral leakages. Variations between injection molded plastic and machined metal thread forms can occur due to different manufacturing processes.

Pipe threads were originally designed as machined thread forms. With the use of thermoplastics and plastic injection molding in the manufacture of plastic pipe thread forms, mold shrinkage and plastic sink make it difficult to insure leak free joints. For this reason, the use of a Teflon based sealant is recommended on all plastic pipe threads. The most common form of sealant is Teflon tape wrapped 2 to 3 turns around the male thread before assembly. Liquid Teflon based sealants are also used successfully to ensure a pressure tight seal. It is always important to use care when applying sealants to avoid introducing the sealant material into the system flow path.

The following sections show examples of how different threads are used and issues that can arise in attempting to create a leak free connection.

When a BSP tapered male thread is tightened into a straight female thread (BSPP) the seal can only be made at the base of the female port with 1 or 2 threads. See figure-14-1. Sealing is compromised by the lack of thread form control in BSP specifications. Variation in crests and roots may cause a mismatch in the thread and create a spiral leak. Thread sealant is required to seal this combination.

Using both tapered male and female BSPT threads would offer a better chance of sealing since you are now matching the taper of the male and female thread. See figure-14-2. This offers more threads a chance of sealing against spiral leakage. Crest and root control is still missing, but with thread sealant, a pressure tight joint would be easier to accomplish.

A number of variations of the NPT thread have been introduced to overcome the problem of spiral leakage and are known as Dryseal threads (See SAE standard J476). The best known is the NPTF (F for Fuel). With this thread design, there are controls on the crests and roots of both the male and the female threads to ensure the crest crushes or displaces material into the root of the mating thread. The interference fit between the crest of one thread and the root of the other along with the thread flanks matching, seals against spiral leakage. Figure 14-3 shows an NPTF male tightened into an NPTF female hand tight. You can see the crest of both the male and female thread flanks meet.

Figure 14-4 shows the NPTF male and female threads tightened approximately 1 turn past hand tight, and you can see the flanks meet and the crests are displaced into the roots.

Although these threads are considered Dryseal, a Teflon tape or liquid is still recommended to aid in the assembly process.

The Teflon works as a lubricant to avoid galling of the material when tightening the two threads together and also fills any voids that may cause leakage.

However, in Nuclear power plants Teflon is not used since its properties deteriorate very fast under radiation conditions.

A variation of the Dryseal thread is the NPSF (National Pipe Straight Fuel). It is used for internal threads and a NPTF external thread can be screwed into it to provide a satisfactory mechanical connection and a hydraulic seal. The combination of a parallel and tapered is not regarded as ideal but is widely used. High quality plastic quick disconnect couplings typically use NPT threads.

Another tapered thread is the British Standard Pipe taper, or BSP, covered by British Standard 21. BSP
thread is commonly used for low pressure plumbing, but is not recommended for medium and high pressure hydraulic systems. This form uses the Whitworth thread with an angle of 55°and a 1 in 16 taper. It is not interchangeable with the American NPT thread, though at the 1/2" and 3/4" size, they both have 14 threads per inch.

Problems arise when threading a NPT male thread form into a BSP female straight thread form. The
1/16”, 1/8”, 1/4”, and 3/8” sizes have a dissimilar pitch, which causes a misalignment of the threads. The flank angles of the threads are also different between NPT and BSP. NPT has a 60° thread where the BSP has a 55° thread.

Figure 14-5 shows a male NPT tightened into a BSPP. Because of the smaller size of the BSPP and the pitch difference, the NPT tightens with only a few turns.

Figure 14-6 shows an NPT tightened into a BSPT. The NPT thread to engage further, but pitch difference eventually causes a binding of the threads. Pitch and thread angle differences will allow spiral leakage.

The 1/2” and 3/4” sizes in the NPT and BSP are all 14 threads per inch, and the NPT will engage the BSP fairly well.

Although these threads are the same pitch and engage well there are still issues with the tread form. The thread angles and the crest and root tolerances being different will allow spiral leakage as shown in figure 14-6. These threads might be used effectively together if an appropriate thread sealant is incorporated.

Many issues arise when plastic quick disconnect couplings, with their corresponding injection plumbed into metal piped hydraulic systems. Leaks and plastic thread form failures may occur if care is not taken. When investigating a metal to plastic pipe joint failure, two factors viz. chemical attack and over tightening, need to be considered. Chemical attack can occur when improper thread sealants are used. Thread sealing is an attempt to block the spiral leak path which occurs when the crests and roots of the thread forms do not match. Anaerobic thread sealants should be avoided when sealing plastic thread forms. These sealants contain chemicals which may attack plastics. Use of a Teflon-based pipe thread sealant is a better choice for plastic threads.

Over tightening of any plastic pipe thread will have adverse affects on the function of the joint. The major difference between plastics and metals is the behavior of polymers. Injection-molded plastic parts continue to deform if they are held under a constant load e.g. creep. Creep is the continued extension or deformation of a plastic part under continuous load. Typically the plastic material in an injection-molded plastic pipe thread form will creep from being over tightened into a female tapered port. The deformation of the part’s internal features can lead to part failure.

14.5 Dry Seal NPTF Threads
Dryseal pipe threads are based on the USA (American) pipe thread; however, they differ from the USA (American) pipe thread in that they are designed to seal pressure tight joints without the necessity of using sealing compounds. To accomplish this some modification of thread form and greater accuracy in
manufacture is required.

The roots of both external and internal threads are truncated slightly more than the crest, i.e. roots have wider flats than the crests, so that metal to metal contact occurs as the crests and the roots coincident with or prior to flank contact, see figure-14-7. Thus as the threads are assembled wrenching, the roots of the threads crush the sharper +crests of the mating threads .This sealing action at both the major and
minor diameters tends to prevent spiral leakage and pressure tight without the necessity of using sealing compounds, provided that the mating threads are in accordance with standard specification and tolerance and are damaged by galling in the assembly. The control of crest and root truncation is simplified by the use of properly designed threading tools. Also it is desirable that both for the length. However, where not functionally objectionable, the use of a compatible lubricant or sealant may be used to minimize the possibility of galling. This is desirable in assembling dryseal pipe threads in refrigeration and other systems to affect a pressure tight seal.

In order to obtain a pressure tight seal using dryseal pipe threads without a sealer, it is necessary to hold crest and truncation of both internal and external threads within the limits specified. Unless this is done by use of threading tools with the crest and root truncation controlled so assure reproduction on the product of threads, it is necessary to use a system of measuring or a system of gauging and measuring to determine conformance.

There are two classes of Dryseal pipe threads viz. Class-I and Class-II Dryseal pipe threads. The classes differ only in inspection requirements. For class-I threads, inspection of roots and crest is not required while for class-Ii threads these inspections are required.

External Dryseal threads are tapered only while internal Dryseal threads may be either straight or tapered. Also, the thread lengths may be either standard or short depending on the requirement of the application. Short threads are obtained by shortening the length of the standard thread by one pitch.

The minimum material condition as shown at the left is established by having the mating crests and roots of equal truncation so as to assure metal to metal contact at these points coincident with flank contact. The condition is established at the sharpest root and the flattest crest and gives no clearance. Tolerances at the crests and the roots are established in the direction of interference only, therefore the maximum material condition shown at the right is established by having the extreme combination of sharpest crests and flattest roots, which provide the maximum interference.

When threaded joints are made wrench tight, it is intended that the flanks and crests and roots shall be in contact.

Figure14-8-: Basic dimensions of NPTF threads


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