Drill Pipe Thread Types, Sizing Equivalents, and Connection Selection
API Tool Joint Connection Equivalents & Sizing Chart
Sourcing replacements for legacy drill string configurations can be challenging due to cross-branding nomenclatures. Under API Spec 7-1, standard Numbered Connections (NC) have completely replaced older Internal Flush (IF) and Full Hole (FH) designations. Review the drill pipe connection interchangeability matrix below to verify your fleet specifications:
| Connection Style | API NC Equivalent | Standard OD (in) | Standard ID (in) | Thread Pitch (TPI) |
| 2-3/8″ IF | NC26 | 3-3/8″ | 1-3/4″ | 4 (0.065″ Flat) |
| 2-7/8″ IF | NC31 | 4-1/8″ | 2-1/8″ | 4 (0.065″ Flat) |
| 3-1/2″ IF | NC38 | 4-3/4″ | 2-1/16″ | 4 (0.065″ Flat) |
| 4″ IF | NC46 | 6-1/4″ | 3-1/4″ | 4 (0.065″ Flat) |
| 4-1/2″ IF | NC50 | 6-1/2″ | 2-3/4″ | 4 (0.065″ Flat) |
Need an instant wholesale quote for custom thread types or dimensions?
Please below specifying your required connection profile (e.g., NC50, NC46, REG), quantities, and target delivery port. Our engineering team will respond with a formal RFQ layout within 2 hours.
What is a drill string?
A drill string is the complete assembly of drill pipe, drill collars and bottom-hole tools that connects the rig to the drill bit and transmits torque, weight on bit and drilling fluid. Its composition and connection types have evolved over time to improve strength, hydraulic efficiency and operational safety in modern oil and gas wells.
The drill string was traditionally composing of a kelly, drill pipe and a bottom-hole assembly (BHA). With the development of top drive technology, the kelly has gradually been phasing out and replaced by a top drive.
Today, a typical drill string consists of:
Drill pipe section – drill pipe and tool joints, sometimes with reamers
- Bottom-hole assembly (BHA) – mainly drill collars, plus stabilizers, shock absorbers, jars, reamers and other downhole tools
The exact composition and connection types used in a drill string depend on the well design, formation conditions and operational requirements.
1.Drill Pipe Connection Interchangeability: basic component of the drill string
Drill pipe is the basic component of the drill string. It is made of seamless steel pipe (wall thickness generally 9–11 mm). Its main functions are to:Transmit torque from the rig to the bit,Transport drilling fluid to the bit and back to surface.
As the well is deepening, more joints of drill pipe are adding to the string, so drill pipe plays a very important role in oil and gas drilling.
1.1 Drill Pipe Connection Interchangeability structure and specifications
A joint of drill pipe consists of the pipe body and two tool joints. There are two main ways to connect the tool joint to the pipe body:
Fine-threaded connection,The pipe body has a fine male thread at both ends.The pipe is connected to a tool joint with matching female threads.This is often callingd “fine thread drill pipe”.
Friction-welded connection,The tool joint is friction welded to the pipe body (butt-welded drill pipe).This type has become the modern standard.Fine-thread drill pipe has been largely phasing out.
To strengthen the connection between the pipe body and the tool joint, the pipe is thickening at both ends (upset). Common upset forms are:IU (internal upset),EU (external upset),IEU (internal–external upset).
According to API, drill pipe is classified into three length ranges:
Range 1 (R1): 5.486–6.706 m (18–22 ft),Range 2 (R2): 8.230–9.144 m (27–30 ft),Range 3 (R3): 11.582–13.716 m (38–45 ft).
The most commonly used drill pipe outside diameters are:
88.9 mm (3 1/2 in).114.3 mm (4 in).127.0 mm (5 in).
Other sizes are also using according to well design and operator preference.
1.2 Drill pipe steel grades and strength
The steel grade of a drill pipe is defined by the minimum yield strength of the steel. According to API, common drill pipe grades include:D, E – lower strength grades used for shallower wells.X-95, G-105, S-135 – high-strength grades for deeper and more demanding wells.
The higher the steel grade:The higher the yield strength.The greater the tensile, torsional and compressive capacity of the pipe.
In drill string design, it is generally recommended to increase drill pipe strength by selecting a higher steel grade rather than simply increasing wall thickness. This helps control weight while still meeting strength requirements.
2.Drill pipe joints and connection types
2.1 Drill pipe tool joints
Drill pipe tool joints are the threading connectors welding to both ends of the pipe body. Each drill pipe joint has:One pin (male) connection,One box (female) connection.with a “thick” API thread to connect individual drill pipe joints together.
During drilling, these connections are repeatedly made up and broken out, and are subjecting to high clamping forces and wear. Therefore:Tool joints have thicker walls and larger outside diameters than the pipe body.Tool joints are made from stronger alloy steel (for example, 35CrMo and similar materials).
For a proper connection, three conditions must be satisfiing:Same pipe size,Same thread type,Matching pin and box.
Different pipe sizes have different joint sizes, and even for the same size, different manufacturers may use different thread series. To standardize this, API specifies series of drill pipe connection types.
2.2 Old API thread types: IF, FH and REG
In the older API standard, drill pipe connections were classified as:IF (Internal Flush),FH (Full Hole / Full Flush),REG (Regular).
Each type has different hydraulic and mechanical characteristics:IF (internal flush),Mainly used for external-upset drill pipe.
The inner diameter of the tool joint is equal to the inner diameter at the upset section and close to the pipe body ID.
Advantages:Good hydraulics and low flow resistance.
Disadvantage:Larger outside diameter.
FH (full hole / full flush)
Mainly used for internal-upset drill pipe.
The tool joint ID equals the ID at the upset section, which is smaller than the pipe body ID.
Hydraulics:Flow resistance is higher than IF.
Strength:Smaller OD than IF and higher overall strength.
REG (regular)
Also used for internal-upset drill pipe.
The tool joint ID is smaller than both the upset ID and the pipe body ID, so there are three different IDs in the pipe.
Hydraulics:Flow resistance is the highest of the three.
Strength:OD is the smallest and strength is the highest.
Typical use:Commonly used for small-diameter drill pipe, reverse-circulation drill pipe, drill bits and fishing tools.
All three thread types use V-shaped threads, but the thread form (top width, taper and pitch) differs for each type.
2.3 NC (Numeric Connection) threads – drill pipe connection interchangeability
With the rapid development of friction-welding drill pipe, fine-thread drill pipe has been replacing by welding drill pipe. The old API connection series had too many different specifications and were inconvenient in the field.
To simplify and standardize, API introduced the NC (Numeric Connection) series.
NC threads are designating by “NC” plus two digits, for example:NC26, NC31, NC38, NC50, etc.
The digits correspond to the nominal diameter of the thread on the reference circle.
For example, NC26 means a coarse thread with a base circle diameter of approximately 2.668 in.
NC threads are V-shapinged with:A flat thread crest of 0.065 in.A rounding root radius of 0.038 in.
The thread form is calling V-0.038R, and is compatible with the earlier V-0.065 thread commonly using for IF and FH connections.
NC connections are increasingly using in the petroleum industry, especially on new drill strings.
Some old API IF, FH and REG connections are still using on legacy strings and tools.
Because NC and old API threads share the same:Pitch diameter,Taper,Pitch,Thread length.
They can often be using interchangeably in practice (within API recommendations), which helps operators upgrade strings gradually.
2.4 Other drill string connections in the field
In addition to drill pipe tool joints, a complete drill string also includes many other threaded connections, such as:
Crossover subs – used to connect different sizes or thread types of pipe
Saver subs and wear joints – protect frequently made-up connections from wear
Kelly / top drive subs – connect the top drive or kelly to the drill string
Drill collar connections and bit subs – connect drill collars, bits and other BHA tools
Fishing tools – special connections for retrieval operations
All of these connections follow the same basic thread standards as drill pipe connections to ensure compatibility across the drill string.
3.Choosing the right drill string and connections
The composition of the drill string and the choice of connection types directly affect:Torsional capacity,Tensile and compressive strength,Hydraulic efficiency,Operating safety and reliability.
In modern land and offshore wells in the Middle East, Russia, Central Asia, South America and Southeast Asia, NC connections have become mainstream, while some IF, FH and REG connections are still using on older strings and tools.
For new projects, it is important to:
Select suitable drill pipe OD, wall thickness and steel grade based on well depth and loads.Choose standardized NC connections where possible.
Match drill pipe, heavy weight drill pipe (HWDP) and drill collars as a complete string.Reduce unnecessary crossovers and special threads.
A well-designed drill string improves drilling efficiency and reduces non-productive time and connection-related failures.
Upgrading to High Torque Drill Pipe Connection Interchangeabilitys for Deepwater Sectors
Standard rotary shouldered connections often reach their torsional limits in extended-reach drilling (ERD) or ultra-deep intervals. To prevent downhole over-torque and catastrophic connection failure, we engineer proprietary high torque drill pipe connections featuring a dedicated double-shoulder design:
Primary + Secondary Shoulder Mechanics:
The secondary internal shoulder provides a mechanical stop that drastically increases the torsional yield strength (up to 40% higher than standard api nc connections).
Streamlined Hydraulics: The flush internal profile minimizes localized pressure drops, maximizing mud flow and mud pulse telemetry efficiency in deep horizontal sections.
100% Interchangeable Profiles:
Optimized to be fully compatible with legacy premium fleets, allowing risk-free integration into your existing drill string composition.
Direct-Factory Drill Pipe Tool Joint Manufacturer & Supplier
Avoid broker markups and unvetted lead times. As an API Spec 7-1 and API Spec 5DP monogrammed manufacturer, we supply heavy-duty tool joints engineered from premium AISI 4145H modified alloy steel.
Anti-Galling Finishing: Standard premium copper plating or heavy manganese phosphating applied to all threads to ensure flawless make-up on the rig floor.
Custom Hardbanding: Extension blocks pre-configured for Arnco 100XT/300XT or Duraband NC wear pads.
Full Certification: Shipped with individual Material Test Reports (MTR), ultrasonic NDT vetting, and total API traceability logs.
Contact our engineers with your procurement requirements, and our team will provide you with the optimal solution and a reasonable price.
FAQ – drill pipe connection interchangeability
Q1: What are the main components of a drill string?
A drill string mainly consists of:Drill pipe with tool joints.Drill collars.
A bottom-hole assembly (BHA) with stabilizers, jars, reamers and other downhole tools
In modern rigs, the traditional kelly at the top of the string is replacing by a top drive.
Q2: Why did NC connections replace many old API thread types?
NC connections simplify the variety of thread types while maintaining strength and compatibility. They provide standardized dimensions and are easier to manage in the field than the older IF, FH and REG series. This reduces the number of different connections that drilling crews must handle.
Q3: How do I choose drill pipe steel grade for a project?
Steel grade should be selected based on:Planned well depth.Expected torque, tension and bending loads.Sour service requirements (H₂S).
Lower grades such as D and E are using for shallower or less demanding wells. Higher grades such as X-95, G-105 and S-135 provide greater strength for deeper and more complex wells.
Q4: Who can I contact for help designing a drill string?
If you need support to select drill pipe sizes, grades and connection types for a specific well, you can contact our engineering team. Based on your well design, load requirements and regional standards, we can recommend a matching combination of drill pipe, HWDP and drill collars with suitable connections.
Need a complete drill string or replacement drill pipe with NC connections for your next project?
You can send us your well parameters and connection requirements, and our engineers will reply with a proposal and quotation.
