Socket Head Cap Screws: DIN 912 Dimensions, Grades and Selection Guide

Last updated: 5 November 2026

Quick answer

A DIN 912 socket head cap screw is a metric screw with a cylindrical head and a hex socket, dimensioned in ISO 4762 and made in China to GB/T 70.1. Three numbers decide whether it fits your design: head diameter (d_k), head height (k) and socket size (s). For M3 they are 5.5 mm, 3.0 mm and 2.5 mm. Length is measured from under the head, and stock screws above about 125 mm are partially threaded, so check the thread length before you assume full thread. Grade selection is an environment decision, not a strength contest: A2-70 for normal indoor and outdoor exposure, A4-80 in chlorides and marine air, 12.9 only when you need strength in a small envelope.

What a socket head cap screw is, and what it is not

“Socket head cap screw” describes the drive and the head shape, not a single standard. The family contains three parts that buyers routinely confuse:

Part	Standard	Head	Head sits
Socket head cap screw	DIN 912 / ISO 4762 / GB/T 70.1	Cylindrical, hex socket	Above the surface, in a counterbore
Button head socket screw	ISO 7380 / GB/T 70.2	Domed, low profile, hex socket	Above the surface
Countersunk socket screw	DIN 7991 / ISO 10642 / GB/T 70.3	90° conical, hex socket	Flush with the surface

All three take a hex key and all three are sold as “socket screws”, but they are not interchangeable: they have different head heights, different load paths and — critically — different length definitions. A DIN 912 screw and a DIN 7991 screw of the same nominal size are not the same part.

The three head styles differ in more than height: the socket is one key size smaller on the low-profile parts, and countersunk length is measured over the head. Side-by-side numbers, and what changes when you swap one for another, are in DIN 912 vs ISO 7380 vs DIN 7991.

DIN 912 is the workhorse of the three. It is used wherever a small head must transmit a high clamping force: machine tools, fixtures, aluminium extrusion frames, pump and valve bodies, automation and robotics, moulds and dies, and any assembly where the fastener sits in a counterbore so the top surface stays flat. The cylindrical head carries a large bearing area for its diameter, and the internal drive lets you tighten it with a key instead of needing clearance around the head for a wrench.

DIN 912 dimensions: the table that answers most questions

Dimensions below are the DIN 912 / ISO 4762 values for the sizes normally stocked. d_k and k are maximum values, t is the minimum socket depth, and s is the hex key size (width across flats).

Thread	Pitch (mm)	d_k max (mm)	k max (mm)	Socket s (mm)	t min (mm)	Typical stocked length
M1.6	0.35	3.00	1.60	1.5	0.7	2.5 – 16
M2	0.40	3.80	2.00	1.5	1.0	3 – 20
M2.5	0.45	4.50	2.50	2.0	1.1	4 – 25
M3	0.50	5.50	3.00	2.5	1.3	5 – 40
M4	0.70	7.00	4.00	3.0	2.0	6 – 50
M5	0.80	8.50	5.00	4.0	2.5	8 – 60
M6	1.00	10.00	6.00	5.0	3.0	10 – 80
M8	1.25	13.00	8.00	6.0	4.0	12 – 100
M10	1.50	16.00	10.00	8.0	5.0	16 – 120
M12	1.75	18.00	12.00	10.0	6.0	20 – 150
M16	2.00	24.00	16.00	14.0	8.0	25 – 200
M20	2.50	30.00	20.00	17.0	10.0	30 – 200

Two of these columns matter more than the rest in practice:

k (head height) is what sets your counterbore depth and your stack-up. If the head is 3.0 mm tall and you cut a 2.5 mm counterbore, the head stands proud and the joint will not seat.
s (socket size) is what your assembly tooling depends on. A 1.5 mm key for M1.6 and M2 is at the practical limit of hand tools — expect to strip sockets on under-tightened or over-tightened micro screws, and plan for a torque-controlled driver on a production line.

All sizes from M1.6 upward are produced in China to GB/T 70.1, which is dimensionally equivalent to DIN 912 and ISO 4762. The full mapping across 34 fastener families is in our GB vs DIN vs ISO comparison chart. Stock DIN 912 in 304 and 316 stainless, plus 12.9 alloy steel, is listed in the DIN 912 socket head cap screw range; the wider family including button head and countersunk is under socket head screws.

How length is measured — and the mistake it causes

For DIN 912, length L is measured from the underside of the head to the end of the screw. The head is not included. This is the same convention as hex bolts (DIN 933) and hex nuts, and it is the convention most engineers expect.

Countersunk screws break the convention. DIN 7991 and ISO 10642 are measured overall, including the head, because the head sits inside the material. So a “DIN 7991 M3 × 10” and a “DIN 912 M3 × 10” are not the same amount of screw below the head:

What you order	Length measured	Length below the head	Thread available in a blind hole
DIN 912 M3 × 10	Under head	10.0 mm	10.0 mm
DIN 7991 M3 × 10	Overall, head included (head ≈1.7 mm)	≈8.3 mm	≈8.3 mm

If you switch a design from cap head to countersunk and keep the same length callout, the screw loses roughly one head height of engagement in the hole. In a blind hole or a thin plate that is the difference between full thread engagement and a stripped thread. Recalculate the length when you change head style — this is the most common specification error we see in RFQs.

Thread length: when a DIN 912 is not fully threaded

Short DIN 912 screws are threaded all the way to the head. Longer ones are not. The standard thread length b is defined as:

L ≤ 125 mm: b = 2d + 6
125 < L ≤ 200 mm: b = 2d + 12
L > 200 mm: b = 2d + 25

Thread	b for L ≤ 125 (mm)	Fully threaded up to L =
M3	12	12
M4	14	14
M5	16	16
M6	18	18
M8	22	22
M10	26	26
M12	30	30
M16	38	38
M20	46	46

Above those lengths the unthreaded shank is a feature, not a defect: it acts as a dowel, resists shear across a joint face, and prevents the thread from fretting against a soft bore. If you actually need thread all the way to the head on a long screw — for a tapped-through aluminium block, for example — say so explicitly on the order, because it is a non-standard variant and should not be assumed.

Grades and materials: what each one is for

Material / grade	Nominal tensile strength	Corrosion resistance	Typical use
A2-70 (304 stainless)	700 MPa	Good outdoors and in fresh water; attacked by chlorides	General machinery, food equipment (dry), outdoor fixtures, electronics
A4-70 (316 stainless)	700 MPa	Good in chlorides, marine atmosphere, many chemicals	Marine, coastal, chemical, medical, water treatment
A4-80 (316 stainless, cold worked)	800 MPa	As A4-70	Same environments with a strength requirement
8.8 (carbon steel, zinc plated)	800 MPa	Poor — plated only	Indoor machinery, dry assemblies, cost-sensitive
10.9 (alloy steel)	1,040 MPa	Poor unless coated	High-load machine building
12.9 (alloy steel, 40Cr)	1,220 MPa	Poor unless coated; plating risk (see below)	Moulds, dies, fixtures, tight envelopes

Strength is not the only axis. Stainless and carbon steel have almost the same stiffness (about 193 vs 200 GPa), so a stainless screw does not make a joint “softer” — but stainless has a much lower fatigue strength in the thread roots and behaves differently under repeated load. The comparison most buyers actually need — where A2-70 loses to 8.8 and where it wins — is set out in A2-70 vs grade 8.8.

The 12.9 plating trap. High-strength steel above roughly 1,000 MPa is susceptible to hydrogen embrittlement if it is electroplated and not properly baked afterwards. A zinc-plated 12.9 screw in a humid or loaded application can fail days after installation, with no warning and no visible corrosion. If you need 12.9 and corrosion protection, specify a mechanically applied coating (zinc flake, Dacromet-type) rather than electroplating, and say so in writing.

The stainless galling trap. Stainless-on-stainless threads can seize during assembly — the oxide layer ruptures and cold-welds the mating surfaces. It happens most on small sizes and with power tools. The fix is a lubricant or anti-seize on the thread, or a mismatched pair (A2 screw into A4 nut). The mechanism and the practical countermeasures are covered in stainless fastener galling.

Clearance holes and counterbores

A DIN 912 screw needs two features in the mating part: a clearance hole for the thread and a counterbore for the head. The table gives ISO 273 medium-series clearance holes and a workable counterbore size. Counterbore diameter should clear d_k by 0.2–0.5 mm, and depth should be k plus 0.2–0.5 mm so the head never bottoms out before the joint closes.

Thread	Clearance hole (ISO 273 medium)	Counterbore Ø	Minimum counterbore depth
M1.6	1.8	3.2 – 3.5	1.8
M2	2.2	4.0 – 4.3	2.2
M2.5	2.7	4.7 – 5.0	2.7
M3	3.4	5.7 – 6.0	3.2
M4	4.5	7.2 – 7.5	4.2
M5	5.5	8.7 – 9.0	5.2
M6	6.6	10.2 – 10.5	6.2
M8	9.0	13.2 – 13.5	8.2
M10	11.0	16.2 – 16.5	10.2
M12	13.5	18.2 – 18.5	12.2
M16	17.5	24.2 – 24.6	16.2
M20	22.0	30.2 – 30.6	20.2

Thread engagement is the other half of the joint. As a working rule, 1 × d of engagement in steel and 1.5–2 × d in aluminium or cast iron develops the screw’s full strength. Beyond about 1.5 × d in steel you gain nothing and only add tapping cost.

How to choose: five decisions in order

Does the head have to sit flush? If yes, you need a countersunk screw (DIN 7991 / ISO 10642), not DIN 912. If a raised head is acceptable, DIN 912 gives you more clamping force per head size.
Is head height the constraint? If the assembly is stacked and there is no room for a 6 mm head on an M6 screw, ISO 7380 button head is lower but carries less preload; the ISO 7380 range is the alternative to check before changing thread size.
What is the environment? Dry indoor: 8.8 or 12.9 zinc plated. Outdoors, washdown, food: A2-70. Chlorides, marine, chemical: A4-70 or A4-80. Do not “upgrade” to stainless for strength — it is usually a downgrade in tensile terms.
Will the joint vibrate? A socket head screw has no locking feature. Use a threadlocker, a nylon-patch screw, or a Schnorr-type washer; do not rely on torque alone.
Is the mating part thin sheet? Below about 1.5 mm, thread engagement is the limit and a rivet nut is usually the better answer. DIN 912 into a thin tapped sheet is a common field failure.

Common mistakes

Keeping the length when switching head style. Countersunk heads are measured overall; cap heads are not. The screw gets shorter in the joint and the thread engagement drops by roughly one head height.
Assuming long screws are fully threaded. Above b = 2d + 6, stock DIN 912 has an unthreaded shank. If you need full thread to the head, specify it.
Under-cutting the counterbore. A 0.3 mm error makes the head bottom out, the joint stays open, and the screw feels “tight” while clamping nothing.
Using A2 stainless in a chloride environment. 304 pits in salt spray and in chlorinated water; the fasteners look fine until they are not. Use A4.
Power-driving stainless without lubrication. Galling welds the screw into the thread and the head twists off at removal.
Specifying zinc-plated 12.9 for a wet application. Hydrogen embrittlement is a delayed, brittle failure with no visible warning.

FAQ

What is the difference between DIN 912 and ISO 4762?

Dimensionally they are equivalent, and both are still used in trade. ISO 4762 is the current international standard; DIN 912 is the German designation that most European drawings still use. In China the equivalent is GB/T 70.1. Any of the three designations will get you the same part from a Chinese supplier, but write all applicable numbers on the purchase order so there is no ambiguity at inspection.

Is a DIN 912 screw fully threaded?

Only up to a length of about 2d + 6 mm. For M6 that is 18 mm, for M8 it is 22 mm, for M10 it is 26 mm. Longer screws have an unthreaded shank and a thread length of b, which is a standard feature that helps locate the joint. If you need thread all the way to the head on a longer screw, it must be specified as a special.

What does 12.9 mean on a socket head screw?

The first number is nominal tensile strength in hundreds of MPa (12 → 1,200 MPa), the second is the yield ratio (0.9 → yield is 90% of tensile, i.e. 1,080 MPa). So 12.9 is stronger than 10.9 (1,040 MPa) and much stronger than A2-70 stainless (700 MPa). Higher grade also means higher notch sensitivity and, for plated parts, higher hydrogen embrittlement risk.

Can I use a DIN 912 screw instead of a hex bolt?

Often yes, with two caveats. A socket head screw has a smaller head than a hex bolt of the same thread, so bearing pressure on the clamped surface is higher — check it on soft materials such as aluminium or plastic. And socket heads are usually partially threaded on longer sizes, which changes the shear behaviour of the joint compared with a full-thread hex bolt.

What hex key size do I need for M3 and M4?

2.5 mm for M3, 3.0 mm for M4. M1.6 and M2 both use 1.5 mm, M2.5 uses 2.0 mm, M5 uses 4.0 mm, M6 uses 5.0 mm, M8 uses 6.0 mm, M10 uses 8.0 mm and M12 uses 10.0 mm. At M1.6 and M2 the socket is small enough that a worn key will round it out — replace keys rather than forcing them.

Do you stock M1.6 and M2 socket head cap screws?

Yes. DIN 912 in 304 and 316 stainless is stocked from M1.6 upward, with no minimum order quantity — single pieces are orderable. Below M1.6 (M1.4, M1.2, M0.8) availability is narrower and usually limited to specific standards and lengths; ask with the exact size and we will confirm what is on the shelf rather than quote a lead time we cannot hold.

What torque should I tighten a DIN 912 to?

There is no single correct number, and any table quoted without its assumptions is misleading. Tightening torque follows T = K × d × F, where F is the target preload and K is the nut factor — roughly 0.12 for lubricated, 0.20 for as-received steel, up to 0.24 for dry zinc-plated. That range alone moves the torque by a factor of two for the same preload. For a critical joint, set preload from the application, then measure torque on the actual parts with the actual lubricant; for general assembly, use the value published by the screw manufacturer for that specific finish and grade.