Counterbore vs Spotface: Definitions, Drawing Symbols, and When to Use Each
Two flat-bottomed recesses, two different jobs — how counterbores hide fastener heads, how spotfaces square up rough surfaces, and how to call out each one correctly on a drawing.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 15 min read
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Quick Answer
A counterbore is a cylindrical, flat-bottomed recess machined to the full height of a fastener head, so a socket head cap screw or bolt sits flush with or below the part surface. A spotface is the same shape cut very shallow — just deep enough to clean up the surface — to give a washer or bolt head a flat, square bearing face on rough cast, forged, or welded material. On drawings, the counterbore uses the ⌴ symbol with diameter and depth; a spotface uses the same symbol (often with an SF notation), and per ASME Y14.5-2009 §1.8.14 the depth may be omitted, meaning machine to minimum cleanup. Neither is a countersink: countersinks are conical and seat flat-head screws.
What Is a Counterbore?
A counterbore is a cylindrical, flat-bottomed enlargement of a hole, machined concentric to it, with vertical walls and a flat floor. The smaller through-hole clears the fastener shank; the larger recess is sized to accept the fastener head so nothing protrudes above the surface. It is the standard way to mount socket head cap screws (Allen bolts), button head screws, and hex bolts when a protruding head would interfere with a mating part, catch on moving components, or simply look wrong on a visible face.
Three dimensions define the feature: the clearance hole diameter (drilled first), the counterbore diameter, and the counterbore depth. For a socket head cap screw, the counterbore diameter clears the screw head with a small margin and the depth equals the head height when you want the head flush — deeper if you want it buried under a plug. Because the head bears on the flat floor, a counterbored joint can be torqued to full specification without wedging into the material, which is why counterbores dominate in machinery, fixtures, dies, and automation equipment. One design constraint: keep at least 1.5× the head height in remaining material under the counterbore so the floor does not bulge or crack under torque. In thin sheet where that is impossible, a countersunk flat-head screw is usually the better answer.
What Is a Spotface?
A spotface is best understood as a very shallow counterbore — typically just deep enough to clean the surface. It does not hide the fastener. Its entire job is to convert a rough, curved, or angled surface around a hole into a flat face perpendicular to the hole axis, so a bolt head, nut, or washer seats evenly and the clamp load is not turned into a bending moment on the fastener.
The classic application is castings and forgings. As-cast surfaces carry draft angles, parting-line steps, and general waviness; a bolt torqued down on that surface clamps on a couple of high points instead of a full annulus, which can loosen the joint or fatigue the bolt. A spotface — often only a fraction of a millimetre to 1–2 mm deep — trues the seat without machining the whole face. The same logic applies to weldments (cleaning up distortion and spatter around mounting holes), hydraulic manifolds (flat sealing faces for fittings), and field repairs where a worn seat needs refreshing. Because the surface varies part to part, spotface depth is usually left to the machinist: the drawing gives the diameter, and the cut runs until the full diameter cleans up.
Counterbore vs Spotface vs Countersink: Side by Side
These three features get confused constantly because they all modify the top of a hole. The distinction is geometric and functional: a counterbore hides a flat-bottomed head, a spotface flattens the surface under a head that stays visible, and a countersink seats a tapered flat head. Specifying the wrong one either leaves the fastener proud of the surface or produces a recess the head cannot seat in at all. For the full treatment of the conical option — angles, sizes, and callouts — see our guide to countersink holes.
| Feature | Counterbore | Spotface | Countersink |
|---|---|---|---|
| Purpose | Recess a fastener head flush with or below the surface | Create a flat, clean bearing face on a rough or uneven surface | Seat a flat-head (tapered) screw flush with the surface |
| Geometry | Cylindrical recess, vertical walls, flat bottom | Shallow cylindrical recess, flat bottom | Conical recess matching the screw head angle |
| Typical depth | Full height of the fastener head (3–24 mm for M3–M24 SHCS) | Minimum cleanup depth — usually a fraction of a millimetre to ~1–2 mm | Set by head diameter and included angle |
| Drawing symbol | ⌴ (C’BORE on older prints) | ⌴ with SF notation (legacy: SF inside the symbol) | ⌵ (C’SINK or CSK) |
| Dimension given | Diameter × depth | Diameter, plus depth or remaining thickness — optional per ASME Y14.5 §1.8.14 | Diameter × included angle |
| Typical seat | Socket head cap screw, button head, hex bolt, dowel | Washer or bolt head on cast, forged, or welded surfaces | Flat head screw or flush rivet |
A quick decision rule for the flat-bottomed pair: if the fastener head must disappear into the part, it is a counterbore; if the head stays on the surface but needs a true seat, it is a spotface. And if the screw has a tapered underside, neither — you want a countersink at 82°, 90°, or 100° depending on the fastener standard.
Drawing Symbols and Dimensioning Rules (ASME Y14.5)
The counterbore symbol is ⌴ — a square open at the top — placed before the recess diameter, with the depth symbol ↧ preceding the depth value. A complete callout is a two-line stack: the drilled hole on the first line, the counterbore on the second. For an M6 socket head cap screw that reads:
⌀6.6 THRU
⌴ ⌀11 ↧6
Drill a 6.6 mm through-hole (ISO 273 normal fit), then counterbore 11 mm diameter × 6 mm deep — flush for a standard M6 SHCS head.
Older prints may spell it out as C’BORE or CBORE — legacy abbreviations for the same symbol. If the functional requirement is the ligament under the recess rather than the recess depth, ASME Y14.5 permits dimensioning the remaining material thickness instead. These symbols are part of the broader hole-callout vocabulary covered in our engineering drawing symbols reference.
The spotface has no symbol of its own in current practice. ASME Y14.5-2009 section 1.8.14 specifies the spotface with the counterbore symbol and a diameter; the depth, or the remaining material thickness, may be omitted — and when it is, the machinist cuts the minimum depth that produces a clean, complete flat face. Drawings made to the older Y14.5M-1994 standard show a dedicated symbol — the ⌴ shape with the letters SF inside — and many shops still annotate SF or SPOTFACE next to the counterbore symbol to remove doubt. The practical takeaway: an explicit depth means counterbore; a diameter alone (or with SF) means spotface to cleanup.
Keep tolerances honest on both features: counterbore diameter and depth are usually non-critical clearance dimensions, while hole position and the flatness of the seat are what function. Our guide to CNC machining tolerances covers what is achievable by default and where tighter callouts actually pay.
Metric Counterbore Size Chart for Socket Head Cap Screws
The chart below covers standard ISO 4762 (DIN 912) socket head cap screws from M3 to M24 — the sizes behind the great majority of industrial assemblies. The clearance holes follow ISO 273 in the close (≈H12) and normal (≈H13) fit classes; the counterbore diameters clear the screw head with working margin, and the depths equal the head heights so the head sits exactly flush. Add 1–2 mm to the depth if the head should sit below the surface.
| Screw Size | Clearance Ø Close Fit (mm) | Clearance Ø Normal Fit (mm) | Counterbore Ø (mm) | Counterbore Depth (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 |
Two cautions. First, the chart is for standard socket head cap screws only — button head (ISO 7380), low-head, and shoulder-head screws have different head dimensions, so check the fastener datasheet before counterboring for anything else. Second, choose the clearance fit deliberately: close fit when the bolts locate the assembly, normal fit as the general default, coarse H14 when you need generous float. For matching drill sizes, pitches, and tap drills, use our thread sizes chart. Spotface diameters are not standardized the same way: size them roughly 1.5–2× the nominal bolt diameter, or simply large enough to clear the washer with margin.
How Counterbores and Spotfaces Are Machined
The classic tool is the piloted counterbore: a flat-bottomed cutter with a pilot pin that rides in the pre-drilled hole, keeping the recess concentric even on a drill press or hand drill. Variants include combined 180° drill-counterbores that drill the hole and cut the recess in one plunge, counterbore-only piloted tools for rework on existing holes, and back-counterboring (reverse spotfacing) tools that feed through the hole and deploy a cutter to face the far side of a plate — the standard trick for flanges you cannot reach from above.
On a CNC mill, a flat-bottom end mill is the everyday choice, especially for non-standard diameters. A helical entry produces a cleaner floor than a straight plunge. The end mill has no pilot, so concentricity comes from the machine — a non-issue on a CNC. Dedicated spotfacing tools exist, but in practice most spotfaces are just shallow passes with whatever flat-bottomed tool is already loaded.
Cutting parameters are conservative compared with drilling. A common shop rule is to run an HSS counterbore at roughly 30% of the RPM of the same-diameter drill, with a steady feed and no pecking. Surface speeds by material:
| Material | HSS Cutting Speed (m/min) | Shop Notes |
|---|---|---|
| Aluminium alloys | 40–60 | High speed, light oil or dry; keep flutes clear of built-up swarf |
| Mild steel | 15–25 | Cutting fluid, steady feed, no pecking |
| Alloy / high-tensile steel | 8–15 | Cobalt (HSS-E) tooling holds edge better at these speeds |
| Stainless steel | 5–10 | Never run dry — work-hardens fast; keep the tool feeding, do not dwell |
| Cast iron | 15–20 | Machine dry; coolant can thermally shock and crack the iron |
Use cutting fluid on steel and stainless (never dry-cut stainless — it work-hardens and kills the edge), run aluminium fast with light lubricant, and machine cast iron dry. If a counterbore comes out off-centre, the cause is almost always a loose pilot, a non-piloted tool used freehand, or a clearance hole that was mislocated to begin with — the recess follows the pilot wherever it sits. At IsoCNC, counterbores and spotfaces are routine features on the milled parts we ship every week; send the drawing through our instant quote and we will flag any callout that needs clarifying before we cut.
FAQ: Counterbore vs Spotface
What is the difference between a counterbore and a spotface?
Both are flat-bottomed cylindrical recesses machined around a hole, but they differ in depth and intent. A counterbore is a full-depth recess sized to swallow the entire fastener head — for a socket head cap screw, a depth equal to the head height — so the head sits flush with or below the surface. A spotface is a shallow cleanup cut, usually a fraction of a millimetre to a millimetre or two, that gives a washer or bolt head a flat, perpendicular bearing face on rough cast, forged, or welded material — not to hide the fastener.
What does a counterbore callout look like on an engineering drawing?
The counterbore symbol is ⌴ — a square open at the top — placed before the counterbore diameter, with the depth symbol ↧ followed by the depth. A typical stacked callout for an M6 socket head cap screw reads: ⌀6.6 THRU on the first line, ⌴ ⌀11 ↧6 on the second — drill a 6.6 mm through-hole, then counterbore 11 mm diameter by 6 mm deep. Older drawings may write C’BORE or CBORE instead of the symbol. If the material thickness remaining under the recess is what matters, that thickness can be dimensioned instead of the depth.
Is a depth dimension required for a spotface?
No — and this is the key difference in how the two features are specified. Per ASME Y14.5-2009 section 1.8.14, a spotface is dimensioned with the counterbore symbol and a diameter, but the depth (or the remaining material thickness) may be omitted. When neither is given, the machinist cuts the minimum depth needed to produce a clean, full-diameter flat face — exactly the point of the operation on an as-cast or as-forged surface that varies part to part. If the depth or remaining thickness does matter, on a thin wall for example, you must state one of them explicitly.
Counterbore vs countersink: which one does my fastener need?
The fastener head decides. Socket head cap screws, button heads, and hex bolts have flat bearing surfaces under the head, so they need the flat floor of a counterbore. Flat-head (countersunk) screws have a conical underside, so they need a matching countersink: 90 degrees for metric screws (ISO 7046/DIN 965), 82 degrees for imperial flat heads, 100 degrees for most aerospace flush fasteners. A counterbored joint is also stronger, because the head bears on a flat face rather than wedging into a taper and cracking the part under torque.
Can I machine a counterbore or spotface with a regular end mill?
Yes — on a milling machine this is common practice, especially for non-standard diameters. A flat-bottom end mill plunged or helically interpolated to depth produces a perfectly good flat-bottomed recess, and on a CNC mill a helical entry gives a cleaner floor than a straight plunge. The trade-off versus a dedicated piloted counterbore tool is concentricity: a piloted tool locates in the pre-drilled hole, so the recess stays concentric even on a drill press or hand drill. An end mill has no pilot, so recess position depends entirely on machine accuracy — fine on a CNC, risky freehand.
How deep should a counterbore be for a socket head cap screw?
The standard rule: counterbore depth equals the head height, so the head lands flush with the surface. For ISO 4762 (DIN 912) metric socket head cap screws the head height equals the nominal thread size — 6 mm for M6, 10 mm for M10, 12 mm for M12 — and standard charts use exactly those depths. Add 1–2 mm if the head should sit below the surface, for example under a plug. Keep at least 1.5 times the head height in remaining material below the floor, or the joint loses strength and the floor can bulge or crack under torque.
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