Inch Socket-Head Cap Screw Counterbore Chart

Counterbore diameters shown are common ASME B18.3-style drill/counterbore values. Flush depth is based on head height and must account for the controlling drawing and manufacturing tolerance.

Metric Socket-Head Cap Screw Counterbore Chart

ISO 4762 defines metric socket-head cap screw head dimensions; DIN 974-1 defines associated metric counterbore manufacturing dimensions. Exact clearance and depth must still follow the controlling drawing.

A counterbore is machined concentrically around a smaller hole so a fastener head, nut, or washer can sit flush with or below the surface. Typical uses include socket-head cap screws, machine assemblies, tooling, fixtures, recessed bolt heads, and recessed nuts or washers.

For conical-recess dimensions and 82°/90°/100° fastener head matching, see the Countersink Size Chart.

A counterbore is deep enough to recess a fastener head. A spotface is usually a shallow machined bearing surface intended to flatten a rough or cast surface and provide even bearing for a bolt head, nut, or washer.

D_cb must be greater than D_head to accommodate head manufacturing tolerance, assembly, tool access, coatings where applicable, and positional variation. Do not use D_cb = D_head as a general manufacturing rule; the applicable standard or drawing should control.

For nominal flush installation, counterbore depth approximately equals head height, but actual production depth may include intentional allowance. Head-height tolerance and drawing requirements must be considered before setting a production depth.

Do not add arbitrary universal extra depth such as +0.010 in. or +0.5 mm unless explicitly identified as a shop practice rather than a standard requirement.

ASME B18.3-2026 is the current inch authority and includes drill/counterbore tables in supporting appendices. Values shown follow established ASME B18.3-style drill/counterbore practice; confirm final dimensions with the current standard and drawing.

ISO 4762 defines head geometry. DIN 974-1 supplies current counterbore manufacturing dimensions for cylindrical-head screws. Clearance-hole classes should follow the applicable assembly standard and drawing.

ASME B18.3-2026 covers socket-head cap screws, shoulder screws, set screws, driving keys, and explicitly includes appendices with drill sizes, counterbore sizes, and dimensional formulas. Use B18.3-2026, not B18.3-2012, as the current edition; older online charts often still cite the older edition.

ISO 4762:2004 remains current after confirmation in 2023. It covers hexagon socket-head cap screws with coarse threads from M1.6 through M64, product grade A. Relevant dimensions include nominal thread diameter, head diameter, head height, and socket dimensions.

DIN 974-1:2008-02 remains current and specifically covers counterbore manufacturing dimensions for hexagon socket-head cap screws and cheese/pan-head screws. DIN 974-1 provides recess dimensions, while ISO 4762 defines the fastener itself. Keep those standards roles separate.

D is greater than d. Use the Clearance Hole Size Chart to select the smaller lower hole.

ASME-based counterbore references distinguish close and normal clearance holes beneath the same counterbore. The counterbore is controlled by the head, while the lower hole changes according to the required assembly fit.

The smaller lower hole can therefore be a clearance hole or a tap-drilled/threaded hole. Do not confuse counterbore diameter with either clearance-hole or tap-drill diameter. See the Tap Drill Chart and Pilot Hole Size Chart.

Socket-head cap screws are commonly counterbored because their cylindrical head has a relatively high profile, and recessed installation provides machinery clearance and a clean surface. Use ASME B18.3 for inch-series screws and ISO 4762 for metric screws.

Button-head screws have wider, much lower, rounded heads, while low-head socket screws have reduced head height. Both can require different counterbore diameters, depths, and loadability checks than a standard SHCS. Tie exact dimensions to the relevant product standard or manufacturer data.

DIN 974-1 specifically covers counterbore manufacturing dimensions for socket-head cap screws, cheese-head screws, and pan-head screws. These head shapes differ substantially from SHCS geometry, so socket-head counterbore dimensions should not be reused without verification.

DIN 974-2 covers counterbores associated with hexagon bolts and nuts in components with clearance holes. Consider wrench/socket access, head or nut across-flats dimension, across-corners envelope, and washer diameter. Do not size these recesses from socket-head tables.

If a washer is inside the recess, counterbore diameter must be greater than washer outside diameter with appropriate clearance. Head clearance does not automatically guarantee washer clearance. See the Washer Size Chart for washer OD data.

For hex bolts and nuts, consider socket outside diameter, wrench access, and extension geometry. Head clearance does not automatically guarantee tool clearance. See the Socket Size Chart and Hex Bolt Size Chart.

Check distance from recess edge to component edge, neighboring holes, walls or bosses, and the surrounding load path. Avoid a universal minimum unless tied to a specific engineering design standard.

For plate thickness T, remaining material thickness equals T minus H_cb. This matters for strength, thread engagement, bending, cracking, and breakthrough. Do not counterbore deeper than the part can support merely to hide the head.

A real counterbore can have tool corner radius, a small bottom radius, or an entrance chamfer. Check head under-radius or fillet clearance, particularly where sharp hole edges could interfere with a socket-head screw fillet in harder materials.

The underside of a socket-head cap screw can have a fillet near the head-to-shank transition. If the lower hole or counterbore corner interferes, the head may not seat flat, the fillet can be damaged, and fatigue performance can suffer. Provide the required edge relief or countersinking where the controlling screw standard or drawing requires it.

Thin Material

If counterbore depth approaches plate thickness, too little material can remain. Consider a lower-profile screw, countersunk screw, external head, thicker boss, or redesign.

Thick Plate and Machine Parts

Thicker components generally provide more recess depth, but rigidity, concentricity, depth control, chip evacuation, and flat bottom geometry must still be checked.

Potential tools include piloted counterbores, end mills, flat-bottom drills, CNC interpolation, and dedicated counterbore cutters. Do not prescribe cutting speed or feed without material, tool, and machine context.

Choose based on the required alignment, rigidity, machine control, and repeatability. ISO and DIN tool standards may be relevant for individual tools, but tooling should not dominate this fastener-reference page.