Direct answer: hexagonal and round drill rods are not two universally interchangeable versions of the same tool. Rod profile is only one part of a top-hammer drill-string specification. The correct choice is controlled first by the rock drill and shank adapter, thread or taper system, rod-end architecture, rig feed and rod handling, then by hole diameter, depth, flushing and application. A buyer should never replace a specified hexagonal rod with a round rod—or the reverse—based only on similar length or thread name.
This guide is for mines, quarries, contractors, distributors and maintenance teams comparing replacement drill steel or standardizing a top-hammer fleet. It focuses on the buyer question behind “hex vs round”: what actually has to match before a different rod profile can be used safely and productively?
1. What Does Hexagonal vs Round Drill Rod Mean?
The terms describe the cross-section of the rod body. A hexagonal rod has six external flats; a round rod has a circular body. That sounds simple, but current OEM catalogs show why profile cannot be treated as an isolated specification.
Sandvik states that its top-hammer rods are offered in both hexagonal and round configurations. Epiroc’s current top-hammer catalog goes further: within its R and T drill-string families it lists hexagonal drifter or extension rods as well as round extension, guide and speed rods. In other words, thread family and rod-body profile are separate variables.
| Buyer question | Hexagonal rod | Round rod | What must be verified |
|---|---|---|---|
| What does the name describe? | Six-sided rod-body profile | Circular rod-body profile | Body profile alone does not identify the complete connection system. |
| Can both appear in threaded top-hammer systems? | Yes | Yes | Check the exact OEM drill-string family and part configuration. |
| Does the same thread name guarantee interchangeability? | No | Rod profile, section, end design, coupling, length, guide/handling and shank configuration still matter. | |
| Can H22 tapered steel be swapped directly for T38/T45 extension rod? | No | These represent different connection architectures and normally different rock-drill/tooling setups. | |
2. The Most Important Rule: Do Not Select Rod Profile Before the Drill String
A top-hammer drill string transfers percussion energy, rotation torque, feed force and flushing through a chain of matched components. The rod therefore has to work with the components on both sides of it—not only with the hole.
Before asking whether hexagonal or round is better, identify:
- rig make and model;
- rock drill model and variant;
- shank adapter or chuck interface;
- thread family or tapered connection;
- male-male, male-female or other rod-end architecture;
- rod body section and required length;
- coupling or integrated female end where applicable;
- bit connection and bit diameter;
- feed length, rod changer, guides and centralizers; and
- air or water flushing arrangement.
If any of those interfaces changes, the decision can become a system conversion rather than a simple rod purchase.
3. Why “Hex for One Job, Round for Another” Is Too Simplistic
Many online explanations reduce the choice to a rule such as “hexagonal rods are for tunneling and round rods are for bench drilling.” That can describe common product patterns, but it is not a reliable procurement rule.
Epiroc’s current T38 data is a useful counterexample. Its T38 system lists hexagonal extension rods from 3,050 to 4,000 mm and also round extension rods from 1,830 to 4,270 mm, plus round guide rods and round speed rods. The same thread family can therefore include more than one body profile and rod architecture. Epiroc’s T45 range, by contrast, is currently listed with round extension, guide and speed rods. These are OEM system definitions—not universal rules for every supplier.
The practical conclusion is stronger than any generic “hex vs round” rule: use the exact rod profile approved for the rock drill, thread system and drilling configuration you are operating.
4. H22 Tapered Drill Steel vs Threaded Extension Rods: Do Not Confuse the Systems
A common source of ordering errors is comparing an H22 tapered rod with a T38 or T45 threaded extension rod as though cross-section were the only difference.
PerfoMax’s current H22 × 108 mm tapered drill rod is specified around an H22 × 108 machine-side interface and an 11° tapered bit connection. That system is used with compatible pneumatic rock drills and taper bits.
PerfoMax’s current T38 extension rods and T45 extension rods are male-male threaded extension rods intended to operate in a threaded top-hammer drill string with matching shank adapter, coupling and bit.
Changing from H22 tapered tooling to a T38 or T45 threaded string is therefore not a “hex-to-round upgrade.” The rock-drill interface, rod connection, coupling architecture and bit connection all have to be re-evaluated.
5. What Changes When Rod Profile Changes?
Even when two candidate rods belong to the same broad top-hammer family, changing body profile can affect several physical interfaces. Do not approve a substitution until these are checked.
| System variable | Why it matters | Buyer check |
|---|---|---|
| Shank adapter / rock drill | The drill string starts with a specific machine-side interface. | Verify rock drill model, shank adapter part number and rod-end thread. |
| Thread family | R32, T38, T45 and other systems are separate connection families. | Match the complete thread designation, not only rod profile. |
| Rod body section | Guides, centralizers and rod handling are designed around physical dimensions. | Confirm approved body shape and section from OEM or supplier drawing. |
| Rod-end architecture | MM extension rods use separate couplings; MF/speed rods integrate a female end. | Specify MM vs MF and the required coupling arrangement. |
| Length | Feed stroke, carousel and rod changer can limit usable rod length. | Confirm actual feed and handling range before changing rod format. |
| Flushing passage | Cuttings removal depends on the complete air/water path. | Check rod bore, shank flushing, bit flushing and available supply. |
| Bit connection | The bottom of the string must match the selected bit family. | Verify thread/taper and bit diameter independently. |
6. Hexagonal vs Round Drill Rod Selection Workflow
- Start with the installed rock drill. Record the rig, rock drill model and exact machine-side tooling configuration.
- Identify the current drill string. Record shank adapter, thread/taper, rod profile, rod-end architecture, coupling and bit.
- Define the reason for changing. Replacement availability, rod handling, hole straightness, breakage, drilling depth and fleet standardization are different problems and should not be solved with the same assumption.
- Check the OEM-approved rod options. If the manufacturer lists both hexagonal and round rods for the system, compare the exact part configurations rather than treating the shapes as generic substitutes.
- Check rig-side clearances and handling. Feed guides, centralizers, carousel pockets and automated handling can make a physically similar rod unusable.
- Check the bottom-hole connection. Confirm coupling and bit compatibility, plus flushing path and bit diameter.
- Lock the quotation by part number or controlled drawing. The RFQ should state profile, section, thread, end type, length and mating components.
7. Buyer Decision Matrix
| Situation | Best next step | Why |
|---|---|---|
| Replacing a proven H22 tapered rod | Stay with the verified H22 × 108 and taper specification unless the whole system is being changed. | H22 machine-side and taper-bit interfaces are the controlling compatibility points. |
| Replacing a T38 or T45 extension rod | Match thread, MM/MF architecture, profile/section and length to the approved drill string. | Thread name alone is incomplete. |
| OEM lists both hex and round rods in one thread family | Use the exact option approved for your rig and application. | Availability in the same family does not mean every rod is interchangeable. |
| Supplier proposes a different profile because stock is available | Request drawing and full compatibility confirmation before accepting. | Inventory convenience is not engineering evidence. |
| Fleet standardization project | Standardize the full drill-string architecture, not only body profile. | Shanks, couplings, rods, bits and spares must remain controlled as a system. |
8. Common Procurement Mistakes
- Ordering by shape only. “Hex rod” or “round rod” is not a complete specification.
- Assuming the thread defines the body. Current OEM ranges show that one thread family can include multiple rod profiles and architectures.
- Treating H22 and T38/T45 as equivalent categories. Tapered and threaded systems use different interfaces.
- Ignoring MM vs MF. Male-male extension rods and male-female speed rods do not use the same coupling arrangement.
- Ignoring feed and rod handling. A rod that matches the thread can still be wrong for the machine.
- Using generic strength claims. Do not assume one profile is always stronger, lighter or more efficient without comparing the actual section, heat treatment and OEM design.
- Changing one component at a time during a conversion. A new rod can create mismatches at the shank, coupling, guide or bit.
9. What to Send in an RFQ
For replacement or conversion work, send enough information for the supplier to verify the complete connection chain:
- rig manufacturer and model;
- rock drill model and variant;
- shank adapter part number or clear photos;
- current rod profile: hexagonal or round;
- rod body section/diameter from drawing or marking;
- thread family or taper specification;
- MM extension rod, MF/speed rod or other end architecture;
- required rod length;
- coupling details;
- bit thread/taper and bit diameter;
- hole diameter, depth and drilling application;
- air or water flushing arrangement; and
- photos, drawings or previous part numbers when interchangeability is required.
If your current tooling is uncertain, review the related PerfoMax guide Tapered vs Threaded Top Hammer Drilling Tools and the Top Hammer Drill String Anatomy before requesting a replacement.
FAQ
Are hexagonal drill rods stronger than round drill rods?
Do not use shape alone to make that conclusion. Strength, stiffness and fatigue life depend on the actual section dimensions, steel, heat treatment, thread design and loading. Compare the OEM or supplier specification for the exact rods.
Can T38 drill rods be hexagonal or round?
Yes, some current OEM T38 systems include both profiles. Epiroc currently lists hexagonal and round T38 rod options. That does not make every T38 rod interchangeable; the exact rock drill, rod type, body section, length and handling arrangement still have to match.
Is H22 the same thing as a T38 drill rod?
No. H22 × 108 commonly identifies a hexagonal machine-side drill-steel format used in tapered tooling systems, while T38 identifies a threaded top-hammer connection family. They belong to different system architectures.
Can I replace a hex rod with a round rod if the thread is the same?
Only when the rock-drill/rig manufacturer or qualified tooling supplier confirms the complete configuration. Same thread is necessary for threaded mating, but it does not prove that the rod body, guides, handling, length, coupling and shank arrangement are correct.
What is the fastest way to avoid ordering the wrong rod profile?
Send the rig and rock-drill model, current shank adapter, rod profile and section, thread or taper, MM/MF architecture, rod length, coupling, bit connection and clear photos. Ask the supplier to confirm the complete string before quotation.
Technical Sources
Next Step: Verify the Whole Drill String, Not Just the Rod Shape
Hexagonal versus round is a useful visual distinction, but it is not the final purchase specification. If you are replacing drill steel or changing a top-hammer configuration, send PerfoMax your rig, rock-drill, shank, current rod, coupling, bit and hole information. The goal is to verify every interface before the order is locked, rather than discovering a mismatch at the drill site.