DIN 912 vs ISO 7380 vs DIN 7991: Which Low-Profile Socket Head Do You Need?
Last updated: 5 November 2026
Quick answer
All three are metric hex-socket screws, and all three are sold as “socket screws”, but they are different parts with different jobs. DIN 912 (ISO 4762, GB/T 70.1) has a cylindrical head about one thread diameter tall — it takes the highest clamp load and sits in a counterbore. ISO 7380 (GB/T 70.2) has a domed head roughly 55% as tall — lower profile, no machining needed, but the smaller socket and thinner head cap the torque you can apply. DIN 7991 (ISO 10642, GB/T 70.3) has a 90° conical head that finishes flush with the surface — and it is the only one of the three whose length is measured including the head. That last difference is the one that breaks assemblies: the same nominal length gives you 3.7 mm less engagement on an M6 countersunk screw than on an M6 cap screw.
The three families, side by side
| Part | Standard | Chinese standard | Head | Head sits | Typical job |
|---|---|---|---|---|---|
| Socket head cap screw | DIN 912 / ISO 4762 | GB/T 70.1 | Cylindrical, hex socket | In a counterbore, above the surface | Maximum preload in a small envelope |
| Button head socket screw | ISO 7380-1 | GB/T 70.2 | Domed, low profile, hex socket | Above the surface, no recess | Low clearance, visible or snag-free fixing |
| Countersunk socket screw | DIN 7991 / ISO 10642 | GB/T 70.3 | 90° conical, hex socket | Flush with the surface | Surfaces that must stay flat and clear |
Two naming traps worth knowing. “DIN 912” and “ISO 4762” are dimensionally identical, so either callout works — but DIN 7991 was withdrawn and replaced by ISO 10642, and the replacement changed the head height at the top of the size range (M16: 8.8 mm under DIN 7991, 9.3 mm under ISO 10642; M20: 10.16 mm vs 11.2 mm). The shank dimensions did not change. If your drawing says DIN 7991 and your supplier quotes ISO 10642, the parts still fit the same hole — but write the standard you actually want on the PO if head height matters to your stack-up.
Length is measured differently — this is where assemblies go wrong
Cylindrical and button heads are measured from under the head down. The countersunk head is measured over the whole part, including the head, because the head disappears into the material. So an M6 × 20 in each family is three different amounts of screw in the joint:
| What you order | Length definition | Head height k (mm) | Shank below head (mm) | Head above surface (mm) |
|---|---|---|---|---|
| DIN 912 M6 × 20 | Under head | 6.00 | 20.00 | 6.00 unless counterbored |
| ISO 7380 M6 × 20 | Under head | 3.30 | 20.00 | 3.30 |
| DIN 7991 M6 × 20 | Overall, head included | 3.72 | 16.28 | 0 (flush) |
Read the third row carefully. If you keep the callout “M6 × 20” and change the head style from cap to countersunk, the screw loses 3.72 mm of thread engagement inside the joint. On a 10 mm tapped plate that is the difference between a properly engaged thread and a stripped hole. This single arithmetic slip accounts for a large share of the “your screws are too short” complaints we see.
The rule: when converting a joint to countersunk, add the head height to the existing length and round up to the next stocked size. 20 mm of engagement behind a flush M6 head means ordering M6 × 24 (24 − 3.72 = 20.28 mm). Going the other way — countersunk to cap head — subtract the head height, or you will bottom out in a blind hole.
Thread length is the second thing the length callout does not tell you. DIN 912 screws above about 125 mm are partially threaded (thread length b ≈ 2d + 6 up to 125 mm, 2d + 12 to 200 mm); shorter screws are threaded almost to the head. ISO 7380 and DIN 7991 stock is normally fully threaded through the range we hold. If your design depends on an unthreaded shank acting as a dowel, cap head is the only one of the three you can specify with confidence.
Head height, socket size and the torque ceiling
The lower the head, the smaller the hex socket that fits inside it — and torque capacity follows the socket, not the thread. Maximum values:
| Thread | 912 dk | 912 k | 912 key s | 7380 dk | 7380 k | 7380 key s | 7991 dk | 7991 k | 7991 key s |
|---|---|---|---|---|---|---|---|---|---|
| M3 | 5.50 | 3.00 | 2.5 | 5.70 | 1.65 | 2.0 | 6.72 | 1.86 | 2.0 |
| M4 | 7.00 | 4.00 | 3.0 | 7.60 | 2.20 | 2.5 | 8.96 | 2.48 | 2.5 |
| M5 | 8.50 | 5.00 | 4.0 | 9.50 | 2.75 | 3.0 | 11.20 | 3.10 | 3.0 |
| M6 | 10.00 | 6.00 | 5.0 | 10.50 | 3.30 | 4.0 | 13.44 | 3.72 | 4.0 |
| M8 | 13.00 | 8.00 | 6.0 | 14.00 | 4.40 | 5.0 | 17.92 | 4.96 | 5.0 |
| M10 | 16.00 | 10.00 | 8.0 | 17.50 | 5.50 | 6.0 | 22.40 | 6.20 | 6.0 |
| M12 | 18.00 | 12.00 | 10.0 | 21.00 | 6.60 | 8.0 | 26.88 | 7.44 | 8.0 |
| M16 | 24.00 | 16.00 | 14.0 | 28.00 | 8.80 | 10.0 | 33.60 | 8.80–9.30 | 10.0 |
All dimensions in mm; dk and k are maximums; s is the hex key across flats. The M16 countersunk head height depends on which edition of the standard you work to (see above).
Two patterns jump out:
- The low-profile families use one key size smaller than a cap head at the same thread. M8 cap head takes a 6 mm key; M8 button head and M8 countersunk both take 5 mm. Because torsional capacity scales roughly with the square of the key across-flats, that is about 30% less torque capacity before the key or socket yields. Assembly crews notice this as cam-out and rounded sockets long before they notice a strength problem.
- Head height falls much faster than thread diameter. M8 cap head is 8 mm tall (one diameter). M8 button head is 4.4 mm. That 3.6 mm is the whole point of the part — and the whole reason it cannot be treated as a drop-in replacement.
ISO 7380’s reduced loadability, and what the “0” prefix means
ISO 7380-1 does not let you claim the full property class on a button head. The standard marks these screws with a leading zero — 08.8, 010.9, 012.9 — because the head, not the thread, limits the assembly. The usual figure quoted is that a 010.9 button head carries about 80% of the load of a normal 10.9 cap screw of the same size.
What that means when you buy: a button head marked “10.9” on a supplier listing may arrive marked 010.9, or may be a screw that is simply not intended for the preload of a real 10.9 joint. Ask which marking is on the head if the joint is load-bearing. If the answer is “010.9”, plan on roughly 20% less clamp force than the class suggests, or move up one size, or move to a cap head.
The failure mode matters more than the number. A cap screw under overload stretches and eventually snaps in the shank — the head stays intact and the failure is visible. A button head under overload tends to shear the dome off around the socket, leaving a threaded stub in the hole with nothing to grip. That is a workshop problem, not just a strength problem.
Countersunk heads: the countersink is part of the joint
DIN 7991 is the right answer whenever the surface has to stay flat — sliding contacts, wiper paths, gasketed faces, aircraft-style skins, or any panel that a customer will run a hand across. It is also the one that most often gets specified without doing the arithmetic:
- The countersink depth is subtracted from your material. An M6 countersunk head is 3.72 mm deep. In a 3 mm sheet you cannot form the recess at all; in a 5 mm sheet you have 1.28 mm of material left under the head. As a rule, keep at least one thread diameter of sound material below the countersink, or the head pulls through under load.
- The hole must be at least dk wide and cut at 90° for metric. M6 needs a 13.44 mm countersink. Using an 82° imperial bit on a metric screw leaves the head resting on its top edge only — it looks seated, wobbles under torque, and never sits flush.
- Edge distance is set by the head, not the thread. An M6 countersunk head is 13.44 mm across, so the hole needs about 6.7 mm of material to the edge, against 3 mm for the thread alone. This is the constraint that kills countersunk screws in narrow flanges.
- The cone centres the part. That is an advantage for locating covers and a disadvantage where you need the joint to tolerate misalignment — the wedge action pulls the two parts into line and can open a gap at the far edge of a thin panel.
Countersunk screws are commonly stocked in 10.9 and 12.9, and a 12.9 countersunk head is a thin, hard wedge bearing on a soft countersink. In aluminium or mild steel, that head will dimple the seat before the thread ever reaches its rated preload. Use a countersunk washer, or accept a lower tightening torque, and the joint will behave.
Grades and materials we actually hold
Grade choice is an environment decision, not a strength contest. Our stock, by part and by count:
| Family | Sizes held | Grades and materials in stock |
|---|---|---|
| DIN 912 | M1.4 – M27 | 12.9 (40Cr alloy) · A2-70 / 304 · A4-70 / 316 · 14.9 (SCM440) · A2-50 (201) · PA66 nylon · TC4 titanium · H65 brass |
| ISO 7380 | M1.6 – M16 | 10.9 (10B21 carbon steel) · A2-70 / 304 · 12.9 (SCM435) · A4-70 / 316 |
| DIN 7991 | M1.6 – M20 | 10.9 (10B21 carbon steel) · A2-70 / 304 · 12.9 (SCM435) · A4-70 / 316 |
Practical notes on that table: A2-70 gives about 700 MPa tensile and is the default for outdoor and washdown equipment; A4-70 / 316 is the marine and chloride answer, at roughly 1.5× the price. 12.9 is not “better stainless” — it is alloy steel, it will rust without a coating, and it is the wrong choice wherever the joint will see weather. Where a design needs strength and corrosion resistance in the same part, the usual answer is to go up one size in A2-70 rather than to specify 12.9 and fight the rust. That trade-off is set out in more detail in A2-70 vs grade 8.8.
Which one for which job
| Situation | Pick | Reason |
|---|---|---|
| Maximum preload in the smallest package; counterbore acceptable | DIN 912 | Full-depth socket, head height = 1d, best bearing area per size |
| Low clearance above the surface, no machining allowed | ISO 7380 | Head height ≈ 0.55d, no recess needed, rounded and snag-free |
| Surface must be flush: slides, covers, gaskets, skins | DIN 7991 | 90° conical head finishes level with the material |
| Thin sheet, aluminium, or painted panels | ISO 7380 + flat washer | Wide domed head spreads load; a countersink would remove the material entirely |
| Vibration, cyclic load, safety-relevant joint | DIN 912 + threadlocker | Deep socket allows real preload, which is what resists self-loosening |
| Frequent disassembly by hand tools | DIN 912 | Biggest key for the thread size; least risk of a rounded socket |
| Visible fastener on a finished product | ISO 7380 | Domed head reads as a design feature rather than a fixing |
| Blind hole with limited depth | DIN 7991 or 7380 | Shorter head leaves more usable thread depth in the joint |
What to put on the RFQ
Nine out of ten queries we answer could be quoted from a single line. The parts that are usually missing:
- Standard, not just the head shape. “Socket head cap screw M6 × 20” is ambiguous; “DIN 912 M6 × 20, A2-70” is a part number.
- Grade or material. 8.8, 10.9, 12.9, A2-70, A4-70 — and for button heads, whether 010.9 marking is acceptable.
- Length basis when you are converting a joint: say “20 mm of thread engagement below a flush head” rather than “M6 × 20”, and the supplier will do the head-height arithmetic for you.
- Finish if it is steel: plain, black oxide, zinc, nickel, electrophoretic black. Stainless needs no coating, only passivation.
- Quantity and target date. Small lots matter here — we ship from one piece, with no minimum order, so a 50-piece trial run and a 50,000-piece release run go through the same process.
The full standard cross-reference — GB/T, DIN, ISO and JIS numbers on one page — is in GB, DIN and ISO fastener standards compared. If you are still at the design stage on a cap screw joint, the dimension tables and counterbore data are in the DIN 912 socket head cap screw guide.
Shop the three families
- DIN 912 socket head cap screws — M1.4 to M27, 12.9, A2-70, A4-70, 14.9, titanium, nylon
- ISO 7380 button head socket screws — M1.6 to M16, 10.9, 12.9, A2-70, A4-70
- DIN 7991 countersunk socket screws — M1.6 to M20, 10.9, 12.9, A2-70, A4-70
FAQ
Is DIN 7991 the same as ISO 10642?
Dimensionally the shank and thread are identical, and both use a 90° head. ISO 10642 replaced DIN 7991, and the head height changed at the larger sizes — M16 is 8.8 mm under DIN 7991 and 9.3 mm under ISO 10642, M20 is 10.16 mm against 11.2 mm. For M12 and below the two are interchangeable in practice. If head height matters, state the standard on the drawing.
Can I replace a DIN 912 with an ISO 7380 of the same length?
Mechanically it fits the same thread and the same clearance hole, and the shank length is the same because both are measured under the head. What changes is the head: 45% lower, one key size smaller, and reduced loadability under the ISO 7380 “0” prefix rules. It is a safe swap for covers, guards and panels. It is not a safe swap for a joint that was designed around 12.9 preload.
Why does my countersunk screw sit below the surface?
Either the countersink is too deep, or the head is a different standard from the bit you cut it with. Check that the recess is 90° for metric screws (82° is imperial), that the countersink diameter is at least the head diameter (13.44 mm for M6), and that you did not cut deeper than the head height (3.72 mm for M6). A 0 to 0.2 mm sub-flush finish is normal; a visible dip is not.
What length do I order to keep 20 mm of engagement with a flush head?
Add the head height to the engagement and round up to the next stocked length. For M6: 20 + 3.72 = 23.72, so order M6 × 24, which gives 20.28 mm below the head. Never order the engagement figure directly — that is the single most common countersunk ordering error.
Do all three use the same hex key?
No. DIN 912 uses a full-size key for the thread; ISO 7380 and DIN 7991 both use one size smaller. An M8 cap screw takes a 6 mm key, an M8 button head or countersunk screw takes 5 mm. Crews that reach for the familiar key usually discover the difference as cam-out on the first screw.
Will a button head fail before the thread does?
It can. The head is thin, the socket walls are short, and the cross-section between the socket bottom and the thread is the weakest part of the screw. Overload typically shears the dome off around the recess instead of stretching the shank. For anything load-bearing, use a cap head or size up.
Are these available in 316 stainless and in small quantities?
Yes — A4-70 / 316 is held across all three families, along with A2-70 / 304, 10.9 and 12.9. There is no minimum order: single pieces, prototype sets and production releases all ship from the same stock. If you want to check a specific size and grade, the ordering guide explains how to send a list, and the no-minimum-order page covers how small lots are handled.