Counterbore drill bits create a flat-bottomed cylindrical recess that allows a fastener head to sit flush with — or below — a surface. If you work with socket head cap screws, you need a counterbore. If you're reading an engineering drawing and see a stepped hole symbol you don't recognise, this guide explains exactly what it means and how to machine it.
This guide covers the counterbore vs countersink decision, metric size charts for socket head cap screws (M3–M24), drawing symbols, tool types, and how to select the right bit for steel, aluminium, and cast iron.
What Is a Counterbore?
A counterbore is a cylindrical flat-bottomed recess machined concentric to an existing hole. It has two diameters: the smaller clearance hole for the bolt shank, and the larger recess sized to accept the fastener head. The walls of a counterbore are vertical and the base is flat — there is no taper.
The purpose is to allow a fastener to sit at or below the surface of the workpiece. This is required when a protruding bolt head would interfere with a mating component, create a safety hazard, or simply needs to be hidden for aesthetic reasons.
The Three Hole Types: Counterbore, Countersink, and Spotface
These three are frequently confused on the workshop floor and on engineering drawings. They are not interchangeable.
| Hole Type | Shape | Bottom Profile | Used For |
|---|---|---|---|
| Counterbore | Cylindrical recess | Flat | Socket head cap screws, button head screws, hex head bolts below surface |
| Countersink | Conical recess | Tapered (60°, 82°, 90°, or 120°) | Flat head (CSK) screws, rivets, deburring |
| Spotface | Very shallow cylindrical recess | Flat | Creating a clean flat seating face on rough or cast surfaces — not for sinking the head |
A spotface is essentially a very shallow counterbore — typically just deep enough to clean the surface and provide a flat bearing face for a washer or bolt head. It does not sink the fastener head below the surface.
Counterbore vs Countersink: Which Do You Need?
The answer comes down to one thing: the geometry of your fastener head.
Head Geometry Determines the Choice
If your fastener has a flat conical underside (a flat head or CSK screw), you need a countersink. The conical recess matches the taper of the head and draws the screw flush as it's tightened.
If your fastener has a flat bottom and cylindrical or hex head (socket head cap screw, button head screw, hex bolt you want to recess), you need a counterbore. The flat base of the recess provides a proper bearing face for the flat underside of the head.
| Fastener Type | Head Shape | Use |
|---|---|---|
| Socket head cap screw (Allen bolt) | Cylindrical, flat base | Counterbore |
| Button head socket screw | Domed, flat base | Counterbore |
| Flat head (CSK) screw | Conical underside | Countersink |
| Pan head / cheese head screw | Flat top, flat base | Counterbore (if recessing) or plain clearance hole |
| Hex bolt (recessed) | Hex head, flat base | Counterbore |
Strength Considerations
A counterbored joint is generally stronger than a countersunk one for the same bolt size. A socket head cap screw in a counterbore can be torqued to its full specification because the cylindrical head bears on a flat face. A countersunk screw relies on the taper of the head wedging into the recess — over-torque and you pull the head into the material, stripping the recess or splitting the screw head.
Material Thickness
Counterbores require adequate material thickness beneath the recess. As a rule of thumb, you need at least 1.5× the socket head height remaining below the counterbore base to maintain joint integrity. In thin sheet metal where this is not possible, a countersunk screw is the practical choice even if a counterbore would otherwise be preferred.
Counterbore Size Chart for Metric Socket Head Cap Screws
The chart below covers ISO 4762 socket head cap screws from M3 to M24, which covers the great majority of industrial assembly and maintenance work. Two fit classes are shown: close fit (tighter clearance, better alignment, for precision assemblies) and normal fit (standard workshop use).
How to Read the Chart
Three dimensions define a counterbored hole for a given screw size:
- Clearance hole diameter (A) — the through-hole for the bolt shank. Drill this first.
- Counterbore diameter (X) — the diameter of the flat-bottomed recess. This must match or slightly exceed the socket head diameter of the screw.
- Counterbore depth (H) — how deep to machine the recess. This equals the socket head height so the top of the head sits exactly flush. Machine slightly deeper if you want the head to sit below the surface.
Metric Counterbore Dimensions — ISO 4762 Socket Head Cap Screws
| Screw Size | Close Fit — A (mm) | Normal Fit — A (mm) | Counterbore Ø — X (mm) | Counterbore Depth — H (mm) |
|---|---|---|---|---|
| M3 | 3.2 | 3.4 | 6.5 | 3.0 |
| M4 | 4.3 | 4.5 | 8.5 | 4.0 |
| M5 | 5.3 | 5.5 | 10.0 | 5.0 |
| M6 | 6.4 | 6.6 | 11.0 | 6.0 |
| M8 | 8.4 | 9.0 | 14.5 | 8.0 |
| M10 | 10.5 | 11.0 | 18.0 | 10.0 |
| M12 | 13.0 | 13.5 | 20.5 | 12.0 |
| M16 | 17.0 | 17.5 | 26.5 | 16.0 |
| M20 | 21.0 | 22.0 | 33.0 | 20.0 |
| M24 | 25.0 | 26.0 | 40.0 | 24.0 |
Dimensions per ISO 4762 and consistent with ASME B18.3 metric series. Clearance hole diameters per ISO 273 (close and normal fit classes). Always verify against your fastener supplier’s actual head dimensions — some aftermarket socket screws vary slightly from the ISO nominal.
When the Chart Does Not Apply
This chart is for ISO 4762 socket head cap screws only. Button head socket screws (ISO 7380), low-head socket screws, and non-standard grades have different head diameters and heights. Always check the screw datasheet if you are counterboring for anything other than a standard socket head cap screw.
Counterbore Symbols in Engineering Drawings
Engineering drawings use standardised symbols to call out counterbored holes. If you're machining from a drawing and see an unfamiliar symbol next to a hole dimension, it's likely one of these.
The Standard Counterbore Symbol
The counterbore symbol is a square with a horizontal line across the bottom — ⌴ (Unicode ⌴). In older drawings you may also see the abbreviation C’BORE or CBORE. The symbol is always followed by the counterbore diameter and depth.
How to Read a Counterbore Callout
A typical counterbore callout reads as a stack of two dimensions:
- Top line: clearance hole diameter and depth (or “THRU” for through-holes)
- Bottom line: counterbore symbol + counterbore diameter × counterbore depth
Example callout for an M6 counterbored hole:
⌀6.6 THRU
⌴ ⌀11 ↓ 6
This means: drill a 6.6 mm through-hole, then counterbore to 11 mm diameter to a depth of 6 mm.
Counterbore vs Countersink Drawing Notation
| Feature | Symbol | Abbreviation | Dimension given |
|---|---|---|---|
| Counterbore | ⌴ | C’BORE / CBORE | Diameter × depth |
| Countersink | ⌵ | C’SINK / CSK | Diameter × included angle |
| Spotface | ⌴ (shallow) | SF / SFACE | Diameter × depth (depth often minimal or not specified) |
| Depth symbol | ↓ | — | Used with all hole types to denote depth |
Types of Counterbore Drill Bits
Not all counterbore tools work the same way. Choosing the wrong type for your setup is the most common cause of off-centre counterbores and chattered finishes.
Piloted vs Non-Piloted Counterbore Bits
A piloted counterbore bit has a small pilot pin at the centre that locates in the pre-drilled clearance hole. The pilot keeps the counterbore concentric with the clearance hole and prevents wandering. This is the correct choice for freehand drilling or when working on a drill press without a precision fixture.
A non-piloted counterbore bit (also called a flat-bottom or end mill style counterbore) has no pilot. It requires the workpiece to be precisely fixtured or machined on a mill to guarantee concentricity. On a hand drill without fixturing, a non-piloted bit will wander.
For maintenance and fabrication work — the majority of AIMS customers — a piloted bit is the practical choice.
180-Degree Counterbore Drill (Combined Drill and Counterbore)
The Bordo 3871 range stocked by AIMS is a 180-degree counterbore drill: a combined tool that drills the clearance hole and counterbore in a single operation. The pilot tip drills the clearance hole, and the larger-diameter cutting section machines the counterbore simultaneously as the tool plunges.
This is the fastest option when you are drilling fresh holes and counterboring in one step. It is not suitable for counterboring an existing hole — for that, use the Bordo 3870 (counterbore only, no pilot drill) or the Sutton Tools C100 DIN373.
Counterbore-Only Bits (No Pilot Drill)
The Bordo 3870 and Sutton Tools C100 DIN373 are counterbore-only tools with a pilot. They locate in a pre-drilled hole and machine only the counterbore recess. Use these when:
- The clearance hole is already drilled and you need to add a counterbore
- You are reworking an existing component
- You need to control clearance hole diameter and counterbore diameter independently
The Sutton Tools C100 is manufactured from cobalt steel (HSS-E) to DIN 373, making it the preferred choice for harder materials including stainless steel and alloy steels.
HSS vs Cobalt Steel Counterbore Bits
| Grade | Suitable Materials | Notes |
|---|---|---|
| HSS (High Speed Steel) | Mild steel, aluminium, brass, plastics, timber | Good all-rounder for general fabrication. Lower cost. |
| HSS-E (Cobalt 5%) | Stainless steel, high-tensile alloys, cast iron, hardened steels | Better heat resistance. Runs at slower speeds but holds edge longer in tough materials. DIN 373 specification. |
For mild steel and aluminium in a standard workshop, HSS is adequate. For stainless steel fastener applications or hardened components, invest in cobalt (HSS-E). Forcing an HSS bit through stainless steel at the wrong speed will blunt it quickly and produce a poor finish.
Reverse (Back) Counterbore Tools
A back counterbore (also called a reverse counterbore) machines a counterbore on the underside of a plate or component — the side you can't access directly with a standard tool. These are specialist tools used in structural steel, pressure vessel, and pipe flange work where both faces need to seat fasteners. Back counterbore kits are available on request — contact the AIMS team for sourcing.
How to Counterbore a Hole
Step 1: Drill the Clearance Hole First
Mark and drill the clearance hole to the correct diameter for your screw size and fit class (refer to the size chart above). Ensure the hole is perpendicular to the surface — a crooked clearance hole will produce a crooked counterbore regardless of how carefully you set up the counterbore tool.
Step 2: Select the Correct Counterbore Tool
Match the counterbore diameter to the screw head diameter from the size chart. If using a piloted tool, the pilot must match your clearance hole diameter. Most Bordo 3870 and 3871 sets are designated by screw size (e.g., “for M6 screw”), which takes the guesswork out of matching pilot to clearance hole.
Step 3: Set the Depth Stop
On a drill press, set the depth stop to the counterbore depth from the chart. If you want the fastener head to sit 1–2 mm below the surface (for example, to be filled with a plug), add that amount to the depth stop setting.
For hand drilling without a depth stop, wrap tape around the counterbore shank at the correct depth as a visual marker.
Step 4: Machine the Counterbore
Run at the appropriate cutting speed for your material (see the table below). Feed firmly but without excessive pressure. A piloted tool should self-locate in the clearance hole — do not force it in sideways. The cut should be smooth; chatter or squealing indicates too high a speed or insufficient feed pressure.
Cutting Speed Reference
| Material | HSS (m/min) | Cobalt HSS-E (m/min) |
|---|---|---|
| Mild steel | 15–25 | 20–30 |
| Alloy steel / high tensile | 8–15 | 12–20 |
| Stainless steel | 5–10 | 8–15 |
| Cast iron | 15–20 | 20–25 |
| Aluminium | 40–60 | 40–60 |
Use cutting fluid on steel and stainless. Aluminium can be run dry or with light oil. Always use cutting fluid on cast iron? No — machine cast iron dry; coolant causes thermal shock and can crack the iron.
Troubleshooting: Off-Centre Counterbores
If the counterbore is not concentric with the clearance hole, the cause is almost always one of three things:
- Pilot too loose: the pilot diameter is smaller than the clearance hole, allowing the tool to walk. Use a close-fit clearance hole or select a piloted tool sized correctly for your hole.
- Non-piloted tool used freehand: without a pilot and without fixturing, wandering is unavoidable. Switch to a piloted bit or use a mill.
- Clearance hole is itself off-centre: the counterbore is machined concentric to the pilot hole. If the pilot hole is in the wrong place, so is the counterbore. Remark and redrill.
Counterbore Tool Selection Guide
By Application
| Application | Recommended Tool | Stocked by AIMS |
|---|---|---|
| Drilling fresh holes + counterboring in one pass (mild steel) | 180° counterbore drill (combined) | Bordo 3871 |
| Counterboring existing holes (mild steel) | Piloted counterbore, HSS | Bordo 3870 |
| Counterboring in stainless or alloy steel | Piloted counterbore, cobalt HSS-E, DIN 373 | Sutton Tools C100 |
| High-volume production or workshop set (M3–M10) | Counterbore set | Bordo 3870-SET or 3871-SET |
By Material
For mild and low-alloy steel, the Bordo HSS range handles the job comfortably at standard workshop speeds. Use cutting oil and maintain a consistent feed rate.
For stainless steel, use the Sutton Tools C100 cobalt HSS-E. Run at reduced speed (half the speed you'd use for mild steel), apply cutting oil liberally, and avoid dwelling — keep the tool moving to prevent work-hardening at the cut face.
For aluminium, HSS runs well at high speed with light oil or dry. Keep flutes clear of swarf build-up, which can weld to the cutting edge in aluminium at high temperatures.
For cast iron, use HSS or cobalt at moderate speed, machine dry, and clear swarf regularly. Cast iron is brittle — use moderate feed pressure and avoid vibration.
Browse AIMS Industrial’s full counterbore range: Counterbore Drill Bits & Sets
Frequently Asked Questions
What is a counterbore used for?
A counterbore creates a flat-bottomed cylindrical recess that allows a fastener head — typically a socket head cap screw — to sit flush with or below the surface of a workpiece. It is used wherever a protruding bolt head would cause interference, create a hazard, or is not acceptable for design reasons.
What is the difference between a counterbore and a countersink?
A counterbore has vertical walls and a flat bottom; it is used for fasteners with flat-bottomed cylindrical heads (socket head cap screws, button heads). A countersink has angled walls forming a cone; it is used for fasteners with a tapered conical head (flat head or CSK screws). The choice is determined entirely by the geometry of the fastener head.
What is the difference between a counterbore and a spotface?
Both produce a flat-bottomed cylindrical recess, but a spotface is very shallow — just deep enough to create a clean flat bearing face on a rough or as-cast surface. A spotface does not sink the fastener head below the surface. A counterbore is deeper and is sized to accept the full height of the fastener head.
How do I work out the correct counterbore diameter and depth?
For metric socket head cap screws (ISO 4762), use the size chart in this article. The counterbore diameter must equal or very slightly exceed the socket head diameter (column X), and the counterbore depth must equal or slightly exceed the socket head height (column H). If you want the fastener to sit below the surface, add the desired recess depth to H.
What is a piloted counterbore bit and do I need one?
A piloted counterbore bit has a small-diameter pilot pin that locates in the pre-drilled clearance hole to keep the counterbore concentric. For drill press and hand drill work, a piloted bit is essential — without it, the larger cutting diameter has no reference to align to and will wander. Only omit the pilot if the workpiece is fixtured on a milling machine with the tool precisely aligned.
What does the counterbore symbol look like on an engineering drawing?
The counterbore symbol is ⌴ — a square with a horizontal line across the bottom. It appears before the counterbore diameter dimension. Older drawings may use the abbreviation C’BORE or CBORE instead. The depth is given after a down-arrow symbol (↓).
Can I use a counterbore drill bit on wood?
Yes, counterbore bits work on timber and engineered wood products. They are commonly used in furniture and joinery to recess bolt heads so they can be plugged for a clean finish. For wood, HSS at high speed works well. Bordo and Sutton counterbore bits are designed primarily for metal but will perform in wood.
What is the difference between a 180-degree counterbore drill and a standard counterbore bit?
A 180-degree counterbore drill (like the Bordo 3871) is a combined tool that drills the clearance hole and machines the counterbore recess in a single plunge. It is the fastest option when starting from solid material. A standard counterbore bit (like the Bordo 3870 or Sutton C100) only machines the counterbore recess — the clearance hole must be pre-drilled. Use a standard counterbore bit when reworking an existing hole.
Should I use HSS or cobalt for counterboring stainless steel?
Cobalt HSS-E (such as the Sutton Tools C100 DIN 373). Stainless steel work-hardens rapidly when cut at incorrect speeds or when the tool dwells without feeding. Cobalt steel holds its cutting edge at higher temperatures than standard HSS and is the correct choice for stainless, high-tensile alloy steels, and any application where HSS bits are burning out prematurely.
What is a reverse counterbore (back counterbore) tool?
A reverse or back counterbore tool machines a counterbore on the underside (blind face) of a plate or component — the face you cannot access directly from above. The tool is fed through the clearance hole, then a cutting head deploys perpendicular to the shank to machine the recess from the back. They are used in structural, pressure vessel, and pipe flange applications. Contact the AIMS team to source back counterbore tooling for your application.
How do I stop a counterbore from going off-centre?
Use a piloted counterbore bit sized correctly for your clearance hole. The most common cause of off-centre counterbores is a pilot pin that is smaller than the clearance hole, allowing the tool to shift. Ensure the clearance hole itself is accurately positioned before counterboring — the counterbore machines concentric to whatever hole the pilot locates in.
Can I counterbore by hand (without a drill press)?
Yes, with a piloted counterbore bit. The pilot locates in the clearance hole and guides the tool to stay concentric. Keep the drill as square to the surface as possible and use a depth marker (tape on the shank) to control depth. A drill press produces more accurate and repeatable results, but for site work and maintenance applications a hand drill with a piloted bit is practical.
AIMS Industrial stocks counterbore drill bits and sets from Bordo and Sutton Tools, available for immediate dispatch across Australia. Browse counterbores and sets or call our team on (02) 9773-0122 for application advice.