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Countersunk Socket Head Screw Dimensions (ISO 10642, M3 to M20)

Guide · Updated October 28, 2026

ISO 10642 covers hexagon socket countersunk head screws, the 90° flat head screws used where nothing can stand above the surface: fixture plates, slide covers, soft jaws and tooling plates. The table gives coarse pitch, theoretical head diameter dk max, actual head diameter dk min, head height k max, nominal hex key size s and minimum socket depth t from M3 to M20. Values agree between the Global Fastener DIN EN ISO 10642 table, the Böllhoff ISO 10642 data sheet and the Intafast EN ISO 10642 sheet (Intafast lists the older DIN 7991 M20 head minimum). Check the current edition or your supplier for critical work.

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Step by step

Read the thread: Measure the screw diameter and check pitch with a thread gage. The ISO 10642 coarse range in this chart runs M3 to M20.
Pick the hex key: Flat heads take the same key as a button head of the same thread: M6 is 4 mm, M8 is 5 mm. A socket head cap screw of the same thread takes one size larger.
Use a 90° countersink: ISO 10642 heads are 90°. An 82° inch countersink leaves the head riding on its outer edge.
Size the countersink diameter: Cut to the theoretical dk max plus a little, about 0.2 to 0.5 mm, so the head sits flush or just below the surface.
Check flush with the screw: Seat a screw and sweep a straightedge or indicator across it. Adjust the countersink depth until the head is 0 to 0.2 mm below flush.
Torque it lower: Countersunk socket screws have reduced loadability. Use less torque than an ISO 4762 cap screw of the same grade.

ISO 10642 countersunk socket screw dimensions (mm)

Columns are: thread size, coarse pitch P, dk theoretical max (sharp 90° cone), dk actual min, k max, hex key s (nominal) and socket depth t min. The real head has a small flat on the edge, so a measured head reads between dk min and dk theoretical. Size the countersink from the theoretical value.

Size | P | dk theo max | dk actual min | k max | s | t min
M3 | 0.5 | 6.72 | 5.54 | 1.86 | 2 | 1.10
M4 | 0.7 | 8.96 | 7.53 | 2.48 | 2.5 | 1.40
M5 | 0.8 | 11.20 | 9.43 | 3.10 | 3 | 1.75
M6 | 1 | 13.44 | 11.34 | 3.72 | 4 | 2.20
M8 | 1.25 | 17.92 | 15.24 | 4.96 | 5 | 2.90
M10 | 1.5 | 22.40 | 19.22 | 6.20 | 6 | 3.50
M12 | 1.75 | 26.88 | 23.12 | 7.44 | 8 | 4.30
M16 | 2 | 33.60 | 29.01 | 8.80 | 10 | 4.80
M20 | 2.5 | 40.32 | 35.40 | 10.16 | 12 | 5.60

Countersink diameter quick list

For a flush head, cut the 90° countersink to about: M3 7.0 mm, M4 9.2 mm, M5 11.5 mm, M6 13.7 mm, M8 18.2 mm, M10 22.7 mm, M12 27.2 mm, M16 34.0 mm, M20 40.7 mm. These are theoretical dk plus about 0.3 mm. Drill the clearance hole first, then countersink.

Flat head vs button head vs cap screw

M6 compared: ISO 10642 flat head 13.44 mm across, 3.72 mm tall and fully sunk, 4 mm key. ISO 7380-1 button head 10.5 mm across, 3.3 mm proud, 4 mm key. ISO 4762 cap screw 10 mm across, 6 mm tall, 5 mm key. The flat head is the only one that leaves a flush surface without a counterbore, but its cone also locates the part, so two flat heads in one plate fight each other if the hole spacing is off.

Worked example: flush M6 screws in a tooling plate

A 12 mm aluminum sub-plate bolts to a vise with M6 screws. Drill 6.6 mm clearance holes, then countersink 90° to 13.7 mm diameter. The countersink depth is about (13.7 − 6.6) ÷ 2 = 3.55 mm from the surface down to the hole edge. Seat a screw and check: the head should sit 0 to 0.2 mm below flush. Use a 4 mm key.

Worked example: why the second screw will not sit flat

A cover has four M5 flat heads. Three sit flush and the fourth rocks. The holes are 0.3 mm off position, and each 90° cone pulls the plate to its own center. Open the clearance holes to 5.5 mm or use counterbored cap screws for location-critical parts, and let dowels do the locating.

Common problems

Head stands proud: countersink too small or 82° inch cutter used. Recut at 90° to the theoretical dk.
Head sinks too deep: countersink too large, reducing bearing in thin sheet. Thin sheet under about 0.7 × k should use a button head instead.
Socket rounds out: wrong key or overtorque. Shallow sockets need a fully seated, good key.
Head breaks off: high load on a reduced-loadability head. Switch to ISO 4762 in a counterbore.

FAQ

What size Allen key fits an M6 flat head socket screw?

4 mm. An M6 socket head cap screw takes 5 mm.

What angle is an ISO 10642 countersink?

90°. Inch flat head socket screws (ASME B18.3) use 82°, so do not mix the cutters.

What diameter should I countersink for an M8 flat head?

The theoretical head diameter is 17.92 mm. Countersink to about 18.2 mm so the head sits flush.

Is ISO 10642 the same as DIN 7991?

Close but not identical. DIN 7991 is withdrawn and some sizes differ, for example the M20 actual head minimum. ISO 10642 replaced it.

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