HDD Thread Connections Guide – Common Threads Used and How to Identify Them

API: The Industry Standard

The American Petroleum Institute (API) sets the most commonly accepted standard for thread connections in HDD. Most HDD drill bits, reamers, and hole openers adhere to this system, making API threads the go-to option across the industry. Within the API standard, the two most commonly used thread styles are:

– API REG (Regular); and
– API IF (Internal Flush).

These thread types are favoured for their durability, strength, and compatibility across rigs and tooling brands.

Despite being standardised, thread identification in the field can still be tricky—especially without callipers, pitch gauges, or reference tools. A mismatch can lead to increased wear, safety issues, and costly downtime. This guide helps HDD contractors understand the differences and accurately identify threads in the field.

Common API Threads in HDD

API REG (Regular) Thread

API REG (Regular) threads are tapered connections specified under the API 5DP standard, originally developed for oilfield use but widely adopted in HDD due to their strength and durability. They feature a robust buttress-style thread with rounded crests and roots, which provides excellent resistance to stress, fatigue, and high torque loads.

REG threads typically have thicker tool joints than other API connections, offering higher tensile strength and increased resistance to bending. While they don’t provide a flush internal diameter—resulting in some restriction to fluid flow—they are a reliable choice for demanding HDD operations, and are commonly found on adapters, housings, & reamers.

API IF (Internal Flush) Thread

API IF (Internal Flush) threads are also part of the API 5DP standard and are designed to maintain a flush internal diameter through the connection. This improves the flow of drilling fluids, reduces turbulence, and supports more efficient fluid dynamics during drilling.

However, to achieve that flush profile, IF threads have thinner tool joints compared to REG threads. This trade-off means that while IF threads improve fluid flow, they generally offer slightly lower tensile strength than REG threads of the same outer diameter. They’re best suited for longer bores or fluid-critical HDD applications, where smooth internal flow is a priority and mechanical loading is more moderate

Other Thread Types

While API threads dominate the HDD landscape, many manufacturers also use proprietary threads tailored for specific rigs or operational advantages. Ditch Witch®, for example, uses EZ Connect threads, and Vermeer® features QuickFire threads. Identifying whether a thread is API or proprietary is a critical first step when selecting tooling or making replacements.

How to Identify Threads in the Field

When working in the field without specialised tools, accurate thread identification is still possible with some basic steps.

First, determine if you’re looking at a pin (male) or box (female) end. The pin end has external threads, while the box end has internal threads. Then, measure the outside diameter (OD) of the thread. For the pin end, this is measured from the outer edge across the centre to the opposite outer edge. On the box end, measure the diameter at the shoulder.

Next, count the threads per inch (TPI) by marking a one-inch span and counting the number of thread peaks. Then measure the thread length—from shoulder to tip on the pin end, or from shoulder to internal stop on the box end.

Lastly, assess the thread shape. REG threads are typically sharp and deep, whereas IF threads have a flatter and smoother profile. Keep an eye out for any branding or distinct design features, as many OEMs manufacture proprietary threads with unique identifiers.

If you’re unsure, using a Thread ID Reference Card—like those recommended by Radius HDD—can help visually match thread profiles quickly and accurately.

Why Proper Thread Matching Matters

Correctly matching thread types ensures proper joint strength, minimises wear, and helps prevent thread galling or separation during drilling. Even within the API system, mismatching threads can lead to costly delays, equipment damage, and safety concerns. Proper identification reduces downtime, increases productivity, and extends the life of your tooling.

Expert Support When You Need It

At JB Trenchless, we specialise in HDD tooling and thread compatibility. Whether you’re unsure what thread you’re working with, need to source replacements, or want advice on matching components, our team is here to help.

Avoid the risks of mismatched threads—reach out to JB Trenchless for the right guidance and support to keep your HDD operations running at full capacity.

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