The socket head cap screw is the machine builder’s default: a cylindrical head, a hex socket, a hardened body in class 12.9 or a stainless A2 / A4. ISO 4762 fixes head diameter at one and a half times the thread and head height at the thread diameter, and the catalogue stocks it from M1.4 to M24 in every length that ends in a round number. Every other length, every reduced head, every odd pitch and every size above M24 is a screw that gets made.
EKINSUN LTD is a custom parts manufacturer that turns socket head cap screws to drawing: a length between the stock steps or beyond them, a reduced or extended head, a fine, left-hand or inch thread, a size above M24, a material the catalogue does not carry, a drilled head for lock wire. The head table below is ISO 4762 as written; the family table lists the low, button and countersunk forms with their stock ranges; the negative-space table marks the border between buying and making.
"M20 x 2.5 cap screws, 12.9, 185 mm long under the head. Stock goes 180 and 200 and the 200 fouls the casting."
Length is the first reason a cap screw is turned; the head and the pitch are the next two.
| Thread | Head diameter | Head height | Hex key | Coarse pitch |
|---|---|---|---|---|
| M3 | 5.5 mm | 3 mm | 2.5 mm | 0.5 |
| M4 | 7 mm | 4 mm | 3 mm | 0.7 |
| M5 | 8.5 mm | 5 mm | 4 mm | 0.8 |
| M6 | 10 mm | 6 mm | 5 mm | 1.0 |
| M8 | 13 mm | 8 mm | 6 mm | 1.25 |
| M10 | 16 mm | 10 mm | 8 mm | 1.5 |
| M12 | 18 mm | 12 mm | 10 mm | 1.75 |
| M14 | 21 mm | 14 mm | 12 mm | 2.0 |
| M16 | 24 mm | 16 mm | 14 mm | 2.0 |
| M20 | 30 mm | 20 mm | 17 mm | 2.5 |
| M24 | 36 mm | 24 mm | 19 mm | 3.0 |
Head diameter is 1.5 × d and head height equals d for every size in the table; the counterbore for a cap screw is sized from the head diameter plus clearance. Key sizes are the ISO 4762 values.
| Form | Standard | Stock range | Stock grades and materials | Notes |
|---|---|---|---|---|
| Socket head cap screw | ISO 4762 / DIN 912 | M1.4–M24 | 12.9 black M1.6–M24; 10.9 and 8.8 zinc M3–M20; A2 / A4 M1.4–M20; brass M3–M12; titanium M3–M12 | knurled or plain head; half thread from M4 |
| Fine-pitch cap screw | ISO 4762 (fine) | M3–M12 | A2 | M8×1, M10×1.25, M12×1.5 |
| Left-hand cap screw | maker series | M4–M16 | 12.9 black; A2 M6–M12 | counter-rotating spindles |
| Low head cap screw | DIN 7984 | M3–M16 | 12.9 black; 8.8 black; A2 | head about two thirds of ISO 4762 height; for shallow counterbores |
| Thin head cap screw | maker series | M2–M10 | A2; zinc steel | thinner still; sheet-metal covers |
| Button head screw | ISO 7380-1 | M1.6–M20 | 12.9 and 10.9 black; A2; brass M3–M12 | domed low head; keys 2 – 8 mm from M3 to M12 |
| Button head with flange | ISO 7380-2 | M3–M8 | A2; 10.9 | flange replaces the washer |
| Countersunk socket screw | ISO 10642 / DIN 7991 | M2–M24 | 12.9 and 10.9 black; A2; brass M3–M10 | 90° countersink; flush face |
| Truss (large flat) socket screw | maker series | M3–M8 | A2 | wide low head, socket drive |
| Socket cap with thread-locking patch | maker series | M2–M25 | 12.9; A2 | 180° or 360° nylon patch |
| Shoulder screw | ISO 7379 | M3–M16 | 12.9 | see shoulder bolts |
| Socket set screw | ISO 4026–4029 | M1.2–M24 | 12.9; A2 | see set screws |
| Grade or material | Tensile class | Where it belongs | Watch |
|---|---|---|---|
| 12.9 black oxide | ISO 898-1 class 12.9 | machine tools, dies, high-preload joints | hydrogen embrittlement risk if plated; black oxide is the normal finish |
| 10.9 zinc or black | class 10.9 | general machinery | the class most zinc-plated cap screws actually are |
| 8.8 zinc | class 8.8 | frames, guards, covers | matches 8.8 hex bolts and class 8 nuts |
| A2-70 stainless | ISO 3506-1 A2-70 | outdoor, food, cleanroom | about class 8 strength; galling with A2 nuts without lubricant |
| A4-80 stainless | ISO 3506-1 A4-80 | marine, chemical | the strongest common stainless class |
| Brass, titanium | — | electrical, decorative, weight-critical | not for high preload; sized for the joint, not the class |
The hex socket is the weak point of the system: a rounded key or a worn socket strips before the thread does. Use the key size in the table, seat it fully, and on a cap screw that will be tightened often specify a Torx socket instead — the same screw with a star recess is a stock item to M20 and a custom one above.
Read the pitch before you order the cap screw. Lay a steel rule along ten thread crests and divide by ten: 10 mm over ten pitches is a 1 mm pitch (M6 coarse), 12.5 mm is M8 coarse, 15 mm is M10 coarse, 17.5 mm is M12 coarse. A reading that is not a round number of millimetres is an inch thread, and a reading that is shorter than the coarse pitch for that diameter is a fine thread. If diameter, pitch and the size range in the table all agree, buy the stock part. If any of the three is off the shelf, that is the one we make. The full measuring routine, including the inch traps →
| What you need | In the catalogue? | What we make |
|---|---|---|
| ISO 4762 cap screw M1.4–M24 in 12.9, 10.9, 8.8 or A2 / A4, standard length | Yes | Buy it from stock. No one-off competes with a cold-formed and heat-treated cap screw. |
| Low head DIN 7984 M3–M16, button ISO 7380 M1.6–M20, countersunk ISO 10642 M2–M24 | Yes | Stocked; buy it. |
| A length between the stock steps, or longer than the longest stock length | No | Turned from bar in 42CrMo4 (class 12.9 territory after heat treatment), A2, 316L or brass, from 1 piece; length to the drawing. |
| A fine, left-hand or inch thread outside the stock sets | No | Single-pointed to your reading. |
| A cap screw above M24 — M27, M30, M36 | No | Turned from bar; see also large bolts and custom bolts. |
| A head of your own diameter or height, a drilled head for lock wire, a cross-drilled shank | No | Machined; see drilled head bolts. |
| A plain shank ground to a fit, a dog point, a reduced thread | No | Turned and ground; see shoulder bolts for the shouldered form. |
| A cap screw in aluminium 6061 or 7075 | Partly | 7075 cap screws are turned on request for weight-critical, low-preload joints and anodised; 6061 is too soft for a cap screw thread, and we say so. |
| Material | Designation | Where it fits here |
|---|---|---|
| Alloy steel class 12.9 | quenched and tempered, black oxide | the stock high-strength cap screw |
| Alloy steel | 42CrMo4 / 1.7225 | turned custom cap screws in 12.9 territory after heat treatment |
| Carbon steel class 8.8 / 10.9 | zinc plated | stock general-purpose cap screws |
| Stainless 304 | 1.4301 / A2-70 | stock stainless cap screws M1.4–M20 |
| Stainless 316L | 1.4404 / A4-80 | stock in the common sizes; turned above |
| Free-machining stainless | 303 / 1.4305 | custom turned cap screws where corrosion duty allows |
| Brass | CW614N / C36000 | stock M3–M12; turned above |
| Titanium | Grade 5 Ti6Al4V | stock M3–M12; turned above |
| Aluminium 7075 | AlZn5.5MgCu | turned light cap screws, anodised, low preload |
| Aluminium 6061 | AlMg1SiCu | not for cap screw threads; the alloy of the parts they clamp |
A 6 mm key. ISO 4762 keys run 2.5 mm for M3, 3 for M4, 4 for M5, 5 for M6, 6 for M8, 8 for M10, 10 for M12, 12 for M14, 14 for M16, 17 for M20 and 19 for M24. Button and countersunk socket screws take one size smaller.
One and a half times the thread diameter, and the head height equals the thread diameter: an M10 has a 16 mm head 10 mm high, an M12 an 18 mm head 12 mm high. The counterbore is sized from that diameter plus clearance.
12.9 is an alloy steel class with about twice the tensile strength of A2-70 stainless, which sits near class 8. Use 12.9 where preload matters and corrosion does not; use A2 or A4 where corrosion matters and the joint is sized for the lower strength.
Yes. Any length under the head, any thread length, and sizes above M24 are turned from 42CrMo4, A2, 316L, brass or titanium bar, from 1 piece, with the head to ISO 4762 or to your own dimensions.
EKINSUN LTD is a custom parts manufacturer that turns socket head, low head, button and countersunk cap screws in alloy steel, 304 and 316L stainless, brass, titanium and aluminium 7075, in any pitch from M3 to M64, from 1 piece, with a quote within 12 hours.
No list price. The drivers are material, heat treatment, the head form, any drilling or grinding, and quantity. A batch of A2 cap screws in an odd length is a short lathe job; a single 12.9-territory M30 screw with a drilled head and a ground shank is priced as the machined part it is. The quote states which.
Thread and pitch, length under the head, thread length, head form, grade or material, finish, quantity. A photo of the old screw on a rule works. From 1 piece, quote within 12 hours.
Socket screws turned to length, head, pitch and grade — from 1 piece, quote in 12 hours.