Detailed Explanation of Drill Pipe Connection Thread Types: How to Choose Between NC, IF, FH, and REG?

In oil and gas drilling operations, Drill pipe connection thread It is the critical link that connects the entire drill string. Threaded connections are the highest‑stress, most critical components of the drill string, responsible for transmitting the immense rotational torque and weight from the surface down to the drill bit at depths of several thousand feet. Selecting an inappropriate threaded connection can lead to thread failure, loss of tools, costly fishing operations, or even catastrophic downhole incidents.

This article provides a systematic overview of the four primary drill pipe connection thread types under API standards—Regular (REG), Internal Flat (IF), Full Hole (FH), and National Code (NC)—to help you understand their differences and appropriate applications.

I. Why are threaded connections so important? 

Drill pipe joint The thread is a helical groove machined on the drill pipe end (pin) and its corresponding connection (box), also known as a rotary shouldered connection. When mated and tightened to torque, they form a mechanical seal capable of withstanding the stresses of the entire drilling operation.

The American Petroleum Institute (API) has established thread standards to ensure that any two components bearing the same thread designation—regardless of manufacturer—can be seamlessly mated and operate reliably. This standardization is a cornerstone of modern drilling operations.

However, different thread types differ fundamentally in their design—some prioritize strength, others prioritize fluid flow, and still others seek a balance between the two. Understanding these differences is essential for selecting the appropriate type.

II. Detailed Explanation of Four Major Thread Types 

1. Regular Style Connection (REG) 

The Regular (REG) thread is a robust, V‑shaped, large‑taper thread and one of the oldest and most reliable designs. Its defining feature is strength—its design prioritizes thick metal cross‑sections, giving it exceptional resistance to torsional and impact loads.

Core features:

The inner diameter of the joint is smaller than that of the drill pipe’s thickened section, which in turn is smaller than the inner diameter of the pipe body.

Drilling fluid exhibits the highest flow resistance, yet it has a small outer diameter and high strength.

Specification range: 2-3/8″ to 8-5/8″, comprising 8 sizes.

Typical applications:

Drill bit connection

Drill collar connection

Bottom Hole Assembly (BHA)

When to choose REG:

When maximum mechanical strength is required—especially at the bottom of the drill string, where stresses are highest and a failure would result in the loss of costly bottomhole assemblies and drill bits—the REG connection comes at the cost of a significantly reduced inner diameter compared to the tool body, thereby limiting fluid flow. Consequently, it is used almost exclusively in BHA applications, where mechanical integrity takes absolute precedence over hydraulic efficiency.

2. Internal-Flush Style Connection (IF) 

The inner-flat (IF) thread is designed to maximize hydraulic efficiency. Its tool joint has the same inner diameter as the drill pipe, providing a smooth, unobstructed flow path for drilling fluid.

Core features:

The inner diameter of the drill pipe joint, the inner diameter of the pipe body, and the inner diameter at the thickened section are essentially identical.

Drilling fluid exhibits low flow resistance, which facilitates the efficient utilization of hydraulic power.

However, the joint has a relatively large outer diameter, making it prone to wear and exhibiting comparatively lower strength.

Using a V-0.065 flat‑top, flat‑bottom triangular thread profile

Specification range: 2-3/8″ to 5-1/2″, available in 6 sizes.

Typical applications:

Extra-Heavy-Duty Drill Pipe (EUE)

Drill string body connection

When to choose IF:

When you need to maximize drill‑bit hydraulic horsepower, IF connections eliminate the “step” found in REG connections that causes fluid turbulence and pressure loss, allowing more energy to be delivered directly to the bit for improved hole cleaning and higher mechanical drilling rates. However, it should be noted that, due to the tendency of IF threads to concentrate stress, API has gradually phased out this thread type in favor of the numerical (NC) designation.

3. Full-Hole Style Connection (FH) 

The full‑bore (FH) thread is specifically designed to maximize fluid flow. Its connector bore diameter matches the inner diameter of the pipe’s thickened section, providing a larger flow passage for drilling fluids.

Core features:

The inner diameter of the fitting equals the inner diameter of the pipe’s thickened section, but is smaller than the pipe’s nominal inner diameter.

The flow resistance of drilling fluid is greater than that of an internal‑flat coupling, but the outer diameter is smaller.

Highest hydraulic efficiency

However, its mechanical strength is lower than that of an IF connector of similar size.

Specification range: 3-1/2″ to 6-5/8″, available in 5 sizes.

Typical applications:

Large-diameter section

Internally thickened or internally and externally thickened drill pipe

When to choose FH:

When fluid flow rate is your primary concern, FH connections deliver the highest hydraulic efficiency. Under current API standards, with the exception of the 5-1/2″ FH size, all other FH sizes have been phased out.

4. Numerical Type (NC — Numbered Connection) 

The Numeric Code (NC) thread is a new joint series introduced by API in 1964, designed to enhance the operational performance of rotary‑shoulder‑type threaded connections.

Core features:

Expressed in terms of the thread major diameter (in inches and tenths of an inch).

The thread profile is V-0.038R (with a flat thread crest of 1.651 mm and a rounded thread root of 0.965 mm).

It outperforms REG, FH, and IF in terms of fatigue resistance.

Some NC connectors are interchangeable with older API‑standard connectors.

Interchangeable correspondence:

Typical applications:

High-strength drill pipe (minimum yield strength of 75,000 PSI or greater)

Drill pipe and drill collar connection

When to choose NC:

The NC series is the API‑recommended preferred thread type. Because its thread form and taper are more optimized, it is recommended for use in new product designs. The NC series has gradually replaced all IF connections specified in the API standard, as well as all FH connections except the 5‑1/2″ FH.

III. Comparative Summary of Four Thread Types

IV. Selection Guide 

1. If you need maximum strength—choose REG.

REG connections are the strongest joints in the drill string, suitable for bottomhole assembly (BHA), drill bits, and drill collars. However, they incur significant fluid‑flow resistance, making them unsuitable for use in the drill pipe body.

2. If you prioritize hydraulic efficiency—consider FH.

FH connections provide the largest fluid flow passage and are suitable for large‑diameter well sections. At present, only the 5‑1/2″ FH size remains in use; all other sizes have been replaced by NC connections.

3. If you require comprehensive performance—choose NC (highly recommended)

The NC series strikes an optimal balance among strength, hydraulic efficiency, and fatigue resistance. It is the API‑recommended preferred thread type and should be given priority in new product designs.

4. If IF has been deprecated, it is not recommended for use in new designs.

Although IF connections offer high hydraulic efficiency, they are prone to stress concentration and have been classified by the API as obsolete joint types.

V. Frequently Asked Questions (FAQ) 

Q1: Are NC threads interchangeable with the old API threads?

Some are compatible. Several models in the NC series share the same pitch circle diameter, taper, thread pitch, and thread length as older API connections, making them interchangeable. For example, the NC38 is interchangeable with the 3-1/2″ IF, and the NC46 is interchangeable with the 4″ IF.

Q2: Why were IF and FH eliminated?

IF threads are prone to stress concentration, resulting in a higher risk of failure in high‑strength drilling environments. FH, on the other hand, has lower mechanical strength than IF connections of comparable size, and the NC series already adequately meets its intended functions. Consequently, API has classified most IF and FH specifications as obsolete couplings.

Q3: What is the most common type of thread found on-site?

At drilling sites, REG and IF are the most common drill pipe thread types, while FH is less commonly found in tool shops. However, in new equipment procurement, the NC series is gradually becoming the mainstream choice.

Q4: Does the thread type affect drilling costs?

Yes. Improper thread selection can lead to fluid‑energy losses, reduced mechanical drilling rates, and even downhole failures. Proper thread selection directly impacts drilling efficiency, safety, and profitability.

Q5: How can the safety of threaded connections be ensured?

Applying the correct make-up torque is the most critical measure for preventing joint failure. The recommended make-up torque should be determined based on 60% of the joint material’s yield strength and the use of a qualified thread sealant.

VI. Summary 

API drill pipe threads are the industry standard for connecting the entire drill string. Understanding the differences among the four main thread types—REG, IF, FH, and NC—is not merely a technical detail; it is fundamental to operational safety, efficiency, and profitability. In new product design, the National Code (NC) thread is the API‑recommended choice.

Need to select the appropriate thread connection type for your drilling project?

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