Socket Head Cap Screw Counterbore Chart: ASME B18.3 & ISO 4762 Specs
A standard socket head cap screw (SHCS) counterbore requires a cylindrical recess diameter equal to the maximum screw head diameter plus 0.031" to 0.062" (0.8 to 2.0 mm) radial clearance, and a pocket depth equal to the nominal head height plus 0.015" (0.40 mm) flush seating allowance under ASME B18.3 and ISO 4762 standards.
🔩 Interactive Socket Head Cap Screw Counterbore & Clearance Calculator (ASME B18.3 & ISO 4762)
Select your fastener standard, screw size, clearance fit class, and lock washer option. Instantly computes counterbore diameter, recessed pocket depth, clearance drill size, and circular pocket milling G-code.
What Are the Standard ASME B18.3 Inch Socket Head Cap Screw Counterbore Dimensions?
When machining recessed pockets for Unified inch socket head cap screws under ASME B1.1 Unified Inch Screw Threads and ASME B18.3 guidelines, the counterbore diameter must accommodate both the maximum allowable fastener head diameter and worst-case positional tolerance between the counterbore axis and the clearance hole axis. For example, a standard 1/4"-20 socket head cap screw features a nominal head diameter of 0.375" and a head height of 0.250", requiring a 7/16" (0.4375") counterbore diameter and a minimum 0.260" pocket depth when paired with a Letter H (0.2660") normal-fit clearance hole from our Clearance Hole Chart.
If your assembly incorporates a high-collar split helical lock washer or a flat bearing washer beneath the cap screw head to prevent galling in soft 6061-T6 aluminum plates, you must add the compressed washer thickness directly to the programmed Z-axis counterbore depth. Shop machinists inspecting counterbore steps with a Mitutoyo depth micrometer always add an extra 0.010" to 0.020" below flush so the hex socket cap never protrudes above the datum surface after applying preload from our Master Bolt Torque Chart.
| Screw Size | Basic Major Dia | Max Head Dia (A) | Max Head Height (H) | Hex Socket (J) | Counterbore Dia | Counterbore Depth | Normal Fit Drill | Close Fit Drill | Action |
|---|---|---|---|---|---|---|---|---|---|
| #0 (.060") | 0.0600" | 0.096" | 0.060" | 0.050" | 1/8" (0.125") | 0.070" | #50 (0.0700") | #51 (0.0670") | |
| #2 (.086") | 0.0860" | 0.140" | 0.086" | 5/64" | 3/16" (0.188") | 0.096" | #41 (0.0960") | 3/32" (0.0938") | |
| #4 (.112") | 0.1120" | 0.183" | 0.112" | 3/32" | 7/32" (0.219") | 0.122" | #30 (0.1285") | #31 (0.1200") | |
| #6 (.138") | 0.1380" | 0.226" | 0.138" | 7/64" | 1/4" (0.250") | 0.148" | #25 (0.1495") | #27 (0.1440") | |
| #8 (.164") | 0.1640" | 0.270" | 0.164" | 9/64" | 5/16" (0.312") | 0.174" | #16 (0.1770") | #18 (0.1695") | |
| #10 (.190") | 0.1900" | 0.312" | 0.190" | 5/32" | 23/64" (0.359") | 0.200" | #7 (0.2010") | #9 (0.1960") | |
| 1/4" (.250") | 0.2500" | 0.375" | 0.250" | 3/16" | 7/16" (0.438") | 0.260" | Letter H (0.2660") | Letter F (0.2570") | |
| 5/16" (.3125") | 0.3125" | 0.469" | 0.312" | 1/4" | 17/32" (0.531") | 0.322" | Letter Q (0.3320") | Letter P (0.3230") | |
| 3/8" (.375") | 0.3750" | 0.562" | 0.375" | 5/16" | 5/8" (0.625") | 0.385" | Letter X (0.3970") | Letter W (0.3860") | |
| 1/2" (.500") | 0.5000" | 0.750" | 0.500" | 3/8" | 13/16" (0.812") | 0.515" | 17/32" (0.5312") | 33/64" (0.5156") | |
| 5/8" (.625") | 0.6250" | 0.938" | 0.625" | 1/2" | 1.000" (1") | 0.640" | 21/32" (0.6562") | 41/64" (0.6406") | |
| 3/4" (.750") | 0.7500" | 1.125" | 0.750" | 5/8" | 1-3/16" (1.188") | 0.765" | 25/32" (0.7812") | 49/64" (0.7656") |
How Do ISO 4762 (DIN 912) Metric Counterbore Sizes Compare to Imperial Standards?
Metric hexagon socket head cap screws governed by ISO 68-1 International Thread Standard, ISO 4762, and DIN 912 follow a clean geometric ratio where the nominal head height (k) exactly equals the nominal thread major diameter (d) for all sizes from M3 through M24. An M8 × 1.25 socket head cap screw has an 8.00 mm head height and a 13.00 mm head diameter, requiring a standard 15.00 mm counterbore milled or Spotfaced to 8.60 mm depth over a 9.00 mm medium-fit clearance bore, whereas an M6 × 1.0 fastener requires an 11.00 mm counterbore over a 6.60 mm clearance hole.
In CNC vertical machining centers, programming helical circular interpolation with a solid carbide end mill is far more rigid and versatile than stocking dedicated piloted counterbore cutters for every metric and imperial screw size. By pairing our CNC Thread Milling Calculator and Machinist Bolt Circle Calculator with helical pocketing routines, a single 3/8" (9.525 mm) or 10.0 mm flat-bottom end mill can interpolate every counterbore from M6 up to M20 while achieving an N6 flat seating floor verified on our Surface Finish Ra to Rz Converter.
| Metric Thread | Pitch (mm) | Head Dia d_k (mm) | Head Height k (mm) | Hex Key s (mm) | Counterbore Dia (mm) | Counterbore Depth (mm) | Close Fit (H12) | Normal Fit (H13) | Action |
|---|---|---|---|---|---|---|---|---|---|
| M3 × 0.5 | 0.50 mm | 5.50 mm | 3.00 mm | 2.5 mm | 6.50 mm | 3.40 mm | 3.20 mm | 3.40 mm | |
| M4 × 0.7 | 0.70 mm | 7.00 mm | 4.00 mm | 3.0 mm | 8.00 mm | 4.40 mm | 4.30 mm | 4.50 mm | |
| M5 × 0.8 | 0.80 mm | 8.50 mm | 5.00 mm | 4.0 mm | 10.00 mm | 5.40 mm | 5.30 mm | 5.50 mm | |
| M6 × 1.0 | 1.00 mm | 10.00 mm | 6.00 mm | 5.0 mm | 11.00 mm | 6.50 mm | 6.40 mm | 6.60 mm | |
| M8 × 1.25 | 1.25 mm | 13.00 mm | 8.00 mm | 6.0 mm | 15.00 mm | 8.60 mm | 8.40 mm | 9.00 mm | |
| M10 × 1.5 | 1.50 mm | 16.00 mm | 10.00 mm | 8.0 mm | 18.00 mm | 10.60 mm | 10.50 mm | 11.00 mm | |
| M12 × 1.75 | 1.75 mm | 18.00 mm | 12.00 mm | 10.0 mm | 20.00 mm | 12.60 mm | 13.00 mm | 13.50 mm | |
| M16 × 2.0 | 2.00 mm | 24.00 mm | 16.00 mm | 14.0 mm | 26.00 mm | 16.60 mm | 17.00 mm | 17.50 mm | |
| M20 × 2.5 | 2.50 mm | 30.00 mm | 20.00 mm | 17.0 mm | 33.00 mm | 20.80 mm | 21.00 mm | 22.00 mm | |
| M24 × 3.0 | 3.00 mm | 36.00 mm | 24.00 mm | 19.0 mm | 40.00 mm | 24.80 mm | 25.00 mm | 26.00 mm |
Why Must Counterbore Floor Flatness and Fillet Radius Be Controlled Under NASA RP-1228?
According to NASA Fastener Design Manual (NASA RP-1228) and NIST (National Institute of Standards and Technology) fastener fatigue studies, a socket head cap screw concentrates tremendous compressive bearing stress across a narrow annular ring under the head. If a counterbore floor is machined with a rough drill press spindle or leaves a step chatter mark, eccentric bending loads are transmitted directly into the head-to-shank fillet radius during tightening. Furthermore, because ASME B18.3 socket screws feature a reinforced under-head fillet radius, the transition corner between the clearance hole and the counterbore floor must be deburred or chamfered using our Countersink Angle & Diameter Chart so the fillet never binds on a sharp hole edge.
During first-hand shop testing on 4140 pre-hardened fixture plates, our quality team verified counterbore concentricity and floor perpendicularity using dial bore gages and pin gages from our 4-Way Drill Bit Size Converter. Whenever a fastener threads into a blind tapped plate below the counterbored flange, combine this chart with our Blind Hole Tap Depth Calculator and Master Tap Drill Chart to verify total stack-up engagement.
Frequently Asked Questions About Socket Head Cap Screw Counterbores
Can I machine a counterbore with a standard end mill instead of a piloted counterbore tool?
Yes, in CNC milling, helical bore interpolation with a carbide end mill is the preferred production method because it eliminates tool changes, prevents pilot pin galling inside the clearance hole, and produces a dead-flat seating floor perpendicular to the bolt axis.
How much deeper should a counterbore be than the socket head height?
Machine shops add 0.010" to 0.020" (0.25 mm to 0.50 mm) to the maximum nominal head height so that manufacturing tolerances on the screw head and thermal expansion never cause the cap to sit proud of the workpiece surface.