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Countersink and Counterbore Basics
Guide · Updated October 27, 2026
Countersinks seat flat-head screws flush. Counterbores sink socket-head cap screws below the surface. Using the wrong angle or depth is the usual mistake.
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Step by step
- Match the angle: Use 82° for inch flat-head screws, 90° for metric, and 100° for some aerospace fasteners.
- Size the counterbore: Use the screw chart's counterbore diameter and depth for the cap screw.
- Spot and drill: Drill the through hole first.
- Cut: Use low RPM and steady feed. Use a piloted counterbore in the drilled hole.
- Inspect: Check that the screw head is flush or below the surface, and check countersink diameter with a gage.
Tips
- Avoid chatter with lower speed and a sharp tool.
- Inspect countersinks: checking a chamfer or countersink.
- Deburr afterward.
Related: checking countersinks, drilling.
Worked example: counterbore for a 1/4-20 socket head cap screw
A 1/4-20 socket head cap screw has a 0.375 in head diameter and 0.250 in head height. A common counterbore is 7/16 in (0.4375 in) diameter, 0.265–0.270 in deep for a flush head, with a 0.266 in (letter H) clearance hole. A flat head 1/4-20 screw uses an 82° countersink; the nominal head diameter is about 0.500 in.
Common problems: chatter in countersinks means too much speed; run about half the drill RPM. Check flushness with a straightedge rather than by eye.
Quick shop check
Example: for a #10 flat head screw (82°), countersink to roughly 0.38 in diameter for flush (ASME B18.6.3 lists the #10 82° flat head at 0.385 in max with a theoretical sharp edge and 0.340 in min with a rounded or flat edge, so real heads fall between those). Measure the countersink diameter with a countersink gage or use a test screw.
FAQ
What angle is a countersink for inch screws?
82° for most inch flat-head screws. Metric uses 90°.
Counterbore vs countersink?
A counterbore is a flat-bottom recess for cap screws. A countersink is conical, for flat-head screws.
Why does my countersink chatter?
Speed too high, or a dull or unsupported tool.
How do I check depth?
A depth mic or calipers, checked on gage blocks. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.
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General shop information only — follow your gage manufacturer's instructions and your quality system.