Understanding Rotary Shouldered Connections: NC, REG, IF and FH Threads Explained

In every drilling operation, the drill string is made up of many components joined together: drill pipe, drill collars, stabilizers, mills, and crossover subs. These parts are connected using rotary shouldered connections. These threaded connections must transmit torque, carry heavy loads, and hold pressure, all while being made up and broken out hundreds of times.

Choosing and inspecting the right connection is essential for safe and reliable drilling. In this blog, JOT Oilfield Services, a trusted oil & gas equipment supply & service company, explains what rotary shouldered connections are and the differences between NC, REG, IF, and FH thread types.

What Is a Rotary Shouldered Connection?

A rotary shouldered connection is a threaded joint used to connect drill string components. It has two parts:

• The pin (male end with external threads)
• The box (female end with internal threads)

When made up, the two shoulders of the connection press tightly together. This shoulder - not the threads alone - carries the load and seals the connection. This is why they are called "shouldered" connections.

These connections are designed to:

• Transmit high torque
• Support heavy axial loads
• Maintain pressure integrity
• Withstand repeated makeup and breakout

Why Connection Type Matters

Using the correct connection ensures:

• Proper fit between components from different suppliers
• Reliable torque transfer
• Reduced risk of connection failure and tool separation
• Longer service life for expensive downhole tools

A wrong or worn connection can lead to fatigue cracks, washouts, and costly fishing operations. This is why oilfield equipment manufacturers follow API standards for all connection types.

Common Rotary Shouldered Connection Types

There are several families of rotary shouldered connections. Each was developed for specific performance needs.

NC - Numbered Connection

The NC connection is the most widely used type in modern drilling.

• The number refers to the pitch diameter (for example, NC26, NC38, NC50)
• Based on the V-0.038R thread form
• Offers a strong balance of strength and torque capacity
• Common on drill pipe, drill collars, and many downhole tools

NC connections are popular because they are reliable, standardized, and interchangeable across suppliers.

REG - Regular

The REG (Regular) connection is one of the oldest connection types still in use.

• Uses a V-0.040 thread form
• Common on drill bits, mills, stabilizers, and crossover subs
• Available in many sizes (for example, 6 5/8 REG, 7 5/8 REG)
• Reliable for high-torque applications

REG connections are often found where tools connect to bits and milling assemblies.

IF - Internal Flush

The IF (Internal Flush) connection is designed for better flow.

• Provides a larger internal bore
• Reduces pressure loss and improves fluid flow
• Good for higher flow rate drilling
• Often equivalent to certain NC sizes (for example, 4 IF is similar to NC50)

IF connections are chosen when maintaining flow efficiency is important.

FH - Full Hole

The FH (Full Hole) connection also focuses on flow performance.

• Offers a large bore for good flow
• Balances flow area with connection strength
• Used in various drill pipe and tool applications

FH connections are a practical choice where both flow and strength are needed.

Connection Comparison Overview

The Importance of Thread Inspection

No matter the connection type, thread quality must be verified. Repeated use causes wear, galling, and fatigue that can lead to failure.

Proper thread inspection includes:

• Thread gauging for pitch diameter and taper
• Profile verification against API specifications
• Lead and thread height measurement
• Visual inspection for damage and galling
• Shoulder condition checks

At JOT, every connection is inspected and gauged to ensure it meets API requirements before the tool leaves the facility.

How JOT Supports Reliable Connections

As a trusted Oil & Gas Equipment Supply & Service Company, JOT Oilfield Services provides:

• Precision machining of rotary shouldered connections
• Thread inspection and gauging services
• Recutting and reconditioning of worn connections
• Manufacturing of drill string stabilizers, crossover drilling tools, and milling tools
• Full quality documentation and traceability

We supply operators across the UAE, Oman, and Saudi Arabia with tools built and inspected to industry standards.

Conclusion

Rotary shouldered connections are the backbone of every drill string. NC, REG, IF, and FH connections each offer different strengths - from balanced performance to improved flow. Understanding these connection types helps operators select the right tools and avoid costly failures.

At JOT Oilfield Services, precision machining, expert thread inspection, and reliable reconditioning ensure every connection performs safely in demanding drilling operations.