DIN 7991Countersunk head (90°)Hexagon socket

DIN 7991 M3 × 20 Countersunk head (90°)

DIN 7991 M3 × 20 is a socket countersunk head screw with a 20 mm nominal length, on a M3 × 0.5 mm metric thread. Flat head version of the socket screw. The 90° cone seats flush in a matching countersink so nothing protrudes above the surface.

DIN 7991 M3 × 20 — Countersunk head (90°)Nominal dimensions in mm · Not to scale

Diameter3 mm

Length20 mm

Pitch0.5 mm

Socket size s2 mm

Head Ø dk6 mm

HeadCountersunk head (90°)

DriveHexagon socket

Dimensions

Standard: DIN 7991
Type: Socket countersunk head screw
Size: M3 × 20
Thread: M3 × 0.5
Length L: 20 mm
Socket size s: 2 mm
Head diameter dk: 6 mm
Head height k: 1.86 mm
Thread length b: 18 mm
Drive: Hexagon socket
Head: Countersunk head (90°)

Nominal values in millimetres as published in DIN 7991. Estimated weight 1.351 g per piece in steel (1.359 g in stainless), calculated from the nominal geometry — real parts vary with tolerance and head chamfers.

Thread data

Full M3 thread page

DesignationM3 × 0.5

Pitch0.5 mm

Nominal Ø3 mm

Pitch Ø2.68 mm

Minor Ø ext2.39 mm

Tap drillØ 2.5 mm

Other lengthsDIN 7991 M3

All M3 specifications

M3 × 6 M3 × 8 M3 × 10 M3 × 12 M3 × 16 M3 × 20 M3 × 25 M3 × 30

Interchangeability

DIN 7991 is catalogued with ISO 10642 as its equivalent designation. Dimensions are compared field by field rather than assumed identical.

Equivalent standardISO 10642Hexagon Socket Countersunk Head Screw ComparisonDIN 7991 vs ISO 10642Field-by-field differences and a substitution verdict

Choosing and fitting this size

DIN 7991 M3 × 20 is a countersunk head (90°) in M3, 20 mm long under the head, on a 0.5 mm coarse pitch. Mating threads should be a matching class nut or a tapped hole of the same nominal size.

Common stock grades are 8.8, 10.9 and 12.9 in steel, plus A2-70 and A4-80 in stainless. Class 10.9 and 12.9 carry roughly 1.25 and 1.5 times the proof load of 8.8, but they also need a properly rated mating nut — a class 8 nut under a 12.9 bolt simply moves the failure into the thread. Zinc-plated 10.9 and above should be baked to avoid hydrogen embrittlement.

For a tapped hole, M3 × 0.5 mm takes a 2.5 mm tap drill, and the external thread's minor diameter is 2.39 mm — that is the section carrying the load, not the nominal M3 figure. Thread engagement of at least one times the nominal diameter is what lets the bolt fail before the thread strips.