DTH Drill Pipe Thread Compatibility: API REG Size Guide

Short answer: DTH drill pipe thread compatibility depends on the exact connection designation, mating pin/box interface, hammer top-sub configuration, and drill-pipe connection. API 2-3/8 Reg and API 2-7/8 Reg are different sizes in the API Regular family and are not directly interchangeable. If the pipe and hammer use different thread sizes, use only a correctly specified crossover or adapter arrangement rather than forcing the threads together.

This guide is for quarry, mining, water-well, foundation, drilling-service, and distributor teams that need to confirm DTH drill pipe compatibility before purchasing a hammer, pipe, top sub, or crossover.

1. DTH Drill Pipe Thread Compatibility: What Must Match?

API Specification 7-2 covers threading and gauging of rotary shouldered connections. API describes preferred rotary shouldered connection designs by styles including Number Connection (NC), Regular (REG), and Full-Hole (FH). In DTH drilling, REG-style connections are widely used on drill pipes and hammer backheads or top subs.

The key purchasing point is that “API Reg” is a connection family, not a complete compatibility description. The size must also be stated. “API 2-3/8 Reg” and “API 2-7/8 Reg” identify different connections. Writing only “API Reg” on an RFQ leaves a critical interface unspecified.

Likewise, the outside diameter of the drill pipe does not by itself define the thread. OEM catalogs show that different pipe outside diameters can use different API REG sizes. The exact product drawing, thread designation, and mating component must be checked.

2. API REG Thread Size Chart: 2-3/8 Reg vs 2-7/8 Reg

Current OEM examples illustrate why the thread should be checked rather than inferred. Sandvik lists 76 mm DTH drill pipes with API 2-3/8 Reg pin and box connections, while its larger pipe entries include API 2-7/8 Reg. Epiroc lists a 2-3/8-inch API Reg top-sub thread on the COP W4 2.0. Other hammer families use still larger backhead threads. The correct thread therefore depends on the exact drill-string design, not a universal hammer-size rule.

3. Are API 2-3/8 Reg and API 2-7/8 Reg Interchangeable?

No. They are separate thread sizes. A connection should only be made when the pin and box are the same specified thread standard and size and the components are intended to mate.

“Potentially yes” is deliberate. Matching the printed thread designation is necessary, but a procurement check should still confirm the actual pin/box ends, shoulder condition, thread damage, and the assembly arrangement. Used drill strings may also contain crossovers or replacement top subs that differ from the original machine configuration.

4. How to Match DTH Drill Pipe Threads to the Hammer Top Sub

Use a connection-chain approach rather than selecting each component independently.

1. Identify the exact hammer. Record brand, model, hammer class, and bit shank. Do not rely on “4-inch hammer” or “5-inch hammer” alone.
2. Identify the installed top sub. Confirm the engraved marking, supplier drawing, quotation, or measured connection. Some hammer families can be supplied with more than one top-sub thread.
3. Identify the drill-pipe connection. Record the complete designation, such as API 2-3/8 Reg, plus which end is pin and which is box.
4. Check the rig-side interface. The upper end of the drill string must also match the rotation head, saver sub, or drive arrangement.
5. Check pipe dimensions and air path separately. Pipe OD, wall thickness, internal passage, and length affect drilling performance and handling, but they do not replace thread identification.
6. Identify every crossover. If a transition is required, specify both ends of the crossover. A label such as “API adapter” is incomplete.
7. Confirm the complete string before shipment. Verify hammer top sub → pipe → additional pipes → saver/drive sub as one chain.

5. Why Pipe OD or Hammer Diameter Is Not Enough to Select the Thread

Two common shortcuts create expensive ordering errors.

Shortcut 1: “My pipe is 76 mm, so I know the thread.” A current PerfoMax 76 mm DTH drill pipe is specified with API 2-3/8 Reg pin × box, and Sandvik also lists 76 mm pipe with that connection. But this does not make “76 mm = 2-3/8 Reg” a universal industry rule. Other manufacturers, special pipe designs, and crossover assemblies can differ.

Shortcut 2: “My hammer is 4-inch, so I know the top sub.” PerfoMax CIR90 can be configured with API 2-3/8 Reg or API 2-7/8 Reg. Epiroc lists API 2-3/8 Reg on a specific 4-inch-class COP W4 2.0, while other hammer series use different backhead connections. Always identify the exact top sub actually being supplied or installed.

The correct procurement question is therefore: What exact thread is on each side of the interface?

6. When Do You Need a Crossover or Adapter Sub?

A crossover is used when two otherwise compatible parts of the drilling system need different connection sizes or styles. For example, a drill pipe with API 2-3/8 Reg cannot directly mate with a top sub that is API 2-7/8 Reg, but a correctly engineered crossover can provide a transition if the rig and drill-string design permit it.

Before ordering a crossover, confirm:

• connection A: exact thread style, size, and pin/box gender;
• connection B: exact thread style, size, and pin/box gender;
• required internal air passage and pressure service;
• outside diameter and wrench-flat clearance;
• overall length and effect on the drill-string setup;
• material/strength suitability for impact, torque, and fatigue loading; and
• whether the hammer or rig manufacturer permits the arrangement.

Do not solve a thread mismatch by partially engaging, forcing, re-cutting in the field, or mixing damaged threads. A connection that appears to “start” by hand is not proof that it has the correct form, shoulder engagement, or load capacity.

7. Six Procurement Mistakes That Cause DTH Thread Mismatches

1. Writing only “API Reg.” Always include the complete size designation.
2. Ordering by hammer diameter only. Hammer class does not uniquely define the top-sub thread.
3. Ordering by pipe OD only. Outside diameter and connection size are separate specification fields.
4. Ignoring pin/box orientation. Two components can have the same thread size yet still not provide the required mating ends in the planned assembly.
5. Assuming a crossover is generic. Both ends and the mechanical/air-flow requirements must be specified.
6. Copying an old purchase order without checking the installed string. Replacement top subs, saver subs, or adapters may have changed the actual field configuration.

8. Field Identification: What to Check Before You Place an Order

If the existing drill string is not well documented, stop guessing and collect evidence. The fastest useful package usually includes:

• clear photos of the hammer top-sub marking and thread;
• clear photos of both ends of the drill pipe;
• hammer brand and exact model;
• pipe outside diameter and length;
• existing thread designation from drawings, packing lists, or previous invoices;
• photos and markings of any crossover or saver sub;
• rig make/model and rotation-head interface; and
• a dimensional drawing or sample component when markings are missing.

When threads are worn, damaged, or contaminated, visual identification alone can be unreliable. Use the manufacturer's drawing, calibrated gauges, or supplier verification rather than estimating from appearance.

9. RFQ Checklist for DTH Drill Pipe and Thread Compatibility

Send the following information with your inquiry so the supplier can verify the connection before quotation:

• rig manufacturer and model;
• DTH hammer manufacturer and exact model;
• hammer top-sub thread: complete designation;
• drill-pipe OD, wall thickness if known, and required length;
• drill-pipe thread: complete designation and pin/box arrangement;
• target hole diameter and depth;
• working air pressure and compressor flow;
• rock type and drilling application;
• crossover/saver-sub details if present;
• quantity and destination; and
• photos or drawings of existing connections when interchangeability is required.

For a current PerfoMax reference, the 76 mm DTH Drill Pipe uses API 2-3/8 Reg pin × box. The CIR90 DTH Hammer can be configured with API 2-3/8 Reg or API 2-7/8 Reg top sub, while the CIR110 DTH Hammer is currently described with API 2-7/8 Reg or API 3-1/2 Reg options. You can also review the DTH Hammer Selection Guide or send the complete interface data through the Request a Quote page.

FAQ

Are API 2-3/8 Reg and API 2-7/8 Reg the same thread?

No. They are different connection sizes within the API Regular style. They should not be directly screwed together.

Can a 2-3/8 Reg drill pipe connect to a 2-7/8 Reg DTH hammer?

Not directly. If the system design permits the transition, use a crossover that is correctly specified for API 2-3/8 Reg on one side and API 2-7/8 Reg on the other, with the correct pin/box orientation and mechanical/air-flow requirements.

Does a 4-inch DTH hammer always use API 2-3/8 Reg?

No. Exact top-sub connections vary by hammer model and supplier. For example, PerfoMax CIR90 is currently configurable with API 2-3/8 Reg or API 2-7/8 Reg. Confirm the actual top sub on the unit being ordered or replaced.

Does a 76 mm DTH drill pipe always use API 2-3/8 Reg?

No universal rule should be assumed. The current PerfoMax 76 mm pipe uses API 2-3/8 Reg, and Sandvik lists 76 mm pipes with the same connection, but procurement should still verify the exact product specification rather than infer thread from OD alone.

What information is most important when ordering replacement DTH drill pipes?

The minimum useful set is the exact hammer model, installed top-sub thread, drill-pipe OD and length, exact pipe thread and pin/box arrangement, rig interface, hole diameter/depth, and photos or drawings when interchangeability with an existing string is required.

Technical References

• American Petroleum Institute — Standards Plan, API Specification 7-2
• API — Specification 7-2 Errata: Rotary Shouldered Connection Styles
• Sandvik — DTH Tools Catalog, Drill Pipe Connection Tables
• Epiroc — COP W4 2.0 DTH Hammer

Next Step: Verify the Complete Connection Chain Before Quotation

The safest DTH purchase is not “a pipe that looks right” or “an API Reg hammer.” It is a documented connection chain in which every interface is named and verified. Send PerfoMax the hammer model, top-sub thread, pipe dimensions and thread, rig interface, hole requirements, and connection photos. That information allows the pipe, hammer, and any required crossover to be checked as one system before the order is finalized.