Names and Standards — What the Screw Is Called and Which Standard Applies
DIN 912 is withdrawn; its sizes live on in ISO 4762 for coarse threads and ISO 12474 for metric fine pitch (M8 to M36, product grade A, the successor of ISO 21269). Inch socket head cap screws follow ASME B18.3 for the head and ASME B1.1 for the UNC and UNF thread. EKINSUN LTD is a custom parts manufacturer that makes fine-pitch and inch socket head cap screws to order, solid or hollow, in 304, 316L and other materials. Catalogues in each market use these names:
| Market | Standard name | Everyday name | How fine and inch threads are written |
|---|---|---|---|
| United States | socket head cap screw (SHCS) | Allen bolt, Allen head cap screw | UNF socket head cap screw; 18-8 stainless = 304 |
| United Kingdom | socket cap screw | cap head Allen bolt, socket screw | UNF socket cap screw; fine pitch; A2-70 |
| Germany, Austria, Switzerland | Zylinderschraube mit Innensechskant (DIN 912 / ISO 4762) | Inbusschraube, Innensechskantschraube | Feingewinde (ISO 12474); UNC / UNF, Zollschrauben; V2A = A2 |
| France, Belgium | vis CHC (tête cylindrique à six pans creux) | vis BTR, vis Allen | pas fin; vis UNC / UNF, visserie en pouces |
| Italy | vite a testa cilindrica con esagono incassato | vite a brugola | passo fine |
| Netherlands | cilinderkopschroef met binnenzeskant | inbusbout | fijne draad |
Need fine-thread and inch socket head cap screws in a size, thread or material no catalogue lists? EKINSUN is a custom parts manufacturer: we make fine-thread and inch socket head cap screws to your drawing, sketch or sample — steel up to class 12.9, 304 or 316L stainless steel — from 1 piece, quoted in 12 hours and shipped duties paid. Already a catalogue size? Ask us too — we may have it in stock, and larger quantities cost less per piece. All custom fasteners →
How the Size Is Written
Inch sizes are written as diameter, threads per inch and length under the head: 1/4-20 × 1/2 is a 1/4 in screw with 20 threads per inch, 1/2 in long under the head. Numbered sizes below 1/4 in run from #0 to #10 (#10 is 0.190 in, 4.83 mm). Metric fine is written M, diameter, pitch and length: M10 × 1.25 × 30. The length is always measured under the head, not overall.
Inch Socket Head Cap Screws — UNC / UNF, Head Diameter, Height and Hex Key
The head does not change with the thread: a 1/4-20 and a 1/4-28 screw share the 3/8 in head, 1/4 in high, and the 3/16 in key. Pitch in mm is 25.4 divided by the threads per inch.
| Size | Diameter in / mm | UNC TPI (pitch mm) | UNF TPI (pitch mm) | Head Ø max in / mm | Head height max in / mm | Hex key in / mm |
|---|---|---|---|---|---|---|
| #0 | 0.060 / 1.52 | — (UNF only) | 80 (0.318) | 0.096 / 2.44 | 0.060 / 1.52 | 0.050 / 1.27 |
| #1 | 0.073 / 1.85 | 64 (0.397) | 72 (0.353) | 0.118 / 3.00 | 0.073 / 1.85 | 1/16 / 1.59 |
| #2 | 0.086 / 2.18 | 56 (0.454) | 64 (0.397) | 0.140 / 3.56 | 0.086 / 2.18 | 5/64 / 1.98 |
| #3 | 0.099 / 2.51 | 48 (0.529) | 56 (0.454) | 0.161 / 4.09 | 0.099 / 2.51 | 5/64 / 1.98 |
| #4 | 0.112 / 2.84 | 40 (0.635) | 48 (0.529) | 0.183 / 4.65 | 0.112 / 2.84 | 3/32 / 2.38 |
| #5 | 0.125 / 3.18 | 40 (0.635) | 44 (0.577) | 0.205 / 5.21 | 0.125 / 3.18 | 3/32 / 2.38 |
| #6 | 0.138 / 3.51 | 32 (0.794) | 40 (0.635) | 0.226 / 5.74 | 0.138 / 3.51 | 7/64 / 2.78 |
| #8 | 0.164 / 4.17 | 32 (0.794) | 36 (0.706) | 0.270 / 6.86 | 0.164 / 4.17 | 9/64 / 3.57 |
| #10 | 0.190 / 4.83 | 24 (1.058) | 32 (0.794) | 0.312 / 7.92 | 0.190 / 4.83 | 5/32 / 3.97 |
| 1/4 | 0.250 / 6.35 | 20 (1.270) | 28 (0.907) | 0.375 / 9.53 | 0.250 / 6.35 | 3/16 / 4.76 |
| 5/16 | 0.313 / 7.94 | 18 (1.411) | 24 (1.058) | 0.469 / 11.91 | 0.312 / 7.92 | 1/4 / 6.35 |
| 3/8 | 0.375 / 9.53 | 16 (1.588) | 24 (1.058) | 0.562 / 14.27 | 0.375 / 9.53 | 5/16 / 7.94 |
| 7/16 | 0.438 / 11.11 | 14 (1.814) | 20 (1.270) | 0.656 / 16.66 | 0.438 / 11.13 | 3/8 / 9.53 |
| 1/2 | 0.500 / 12.70 | 13 (1.954) | 20 (1.270) | 0.750 / 19.05 | 0.500 / 12.70 | 3/8 / 9.53 |
Head dimensions are ASME B18.3 maxima. Every size in the table is made to order in UNC or UNF, in 304 (18-8, A2-70) or 316L, solid or hollow, including the fine threads below #10 (#2-64, #3-56, #4-48, #5-44, #6-40, #8-36).
Metric Fine-Pitch Socket Head Cap Screws — ISO 12474
A fine-pitch cap screw has the same head as the coarse one of the same diameter; only the thread changes. The second column is the coarse pitch, so a reading shorter than it on the ten-thread check means a fine thread.
| Fine thread | Coarse pitch, same size | Head Ø dk | Head height k | Hex key s | Ten threads measure |
|---|---|---|---|---|---|
| M8 × 1 | 1.25 | 13 | 8 | 6 | 10.0 |
| M10 × 1 | 1.5 | 16 | 10 | 8 | 10.0 |
| M10 × 1.25 | 1.5 | 16 | 10 | 8 | 12.5 |
| M12 × 1.25 | 1.75 | 18 | 12 | 10 | 12.5 |
| M12 × 1.5 | 1.75 | 18 | 12 | 10 | 15.0 |
| M14 × 1.5 | 2.0 | 21 | 14 | 12 | 15.0 |
| M16 × 1.5 | 2.0 | 24 | 16 | 14 | 15.0 |
| M18 × 1.5 | 2.5 | 27 | 18 | 14 | 15.0 |
| M20 × 1.5 | 2.5 | 30 | 20 | 17 | 15.0 |
| M20 × 2 | 2.5 | 30 | 20 | 17 | 20.0 |
Heads: ISO 4762 / ISO 12474 maxima; M18 × 1.5 to DIN 912. M7 × 0.75 is the ISO 261 fine pitch for M7, a size outside ISO 12474; we make it as well. Every size and pitch in the table is made to order in 304 (A2-70) or 316L, from M7 to M20 and larger to your drawing. The coarse-thread ISO 4762 range is on socket head cap screws.
Solid or Hollow — Cap Screws With a Bore Along the Axis
The same screws are made hollow: a bore runs along the axis, from the bottom of the hex socket to the thread end. It passes a cable, an air or oil line or a smaller screw, or vents a blind hole. The bore can be plain or tapped, as in the photo; its diameter sets how much section is left to carry the preload, so it is chosen with the load in mind. Hollow cap screws usually have a fine thread, because the shallower thread leaves more wall around the bore.
Measure Before You Order: Inch, Metric Coarse or Metric Fine
Take two readings: the outside diameter with a caliper, and the length of ten threads with a rule. A diameter that is not a round number of millimetres points to an inch screw; a ten-thread length shorter than the coarse pitch times ten points to a fine thread. The pairs below are the ones that get swapped:
| Inch / metric | Outside Ø (mm) | Ten threads (mm) | What tells them apart |
|---|---|---|---|
| #10-32 UNF / M5 × 0.8 | 4.83 / 5.00 | 7.94 / 8.00 | diameter: 0.17 mm apart |
| 1/4-20 UNC / M6 × 1 | 6.35 / 6.00 | 12.70 / 10.00 | diameter and pitch |
| 1/4-28 UNF / M6 × 0.75 | 6.35 / 6.00 | 9.07 / 7.50 | diameter and pitch |
| 5/16-18 UNC / M8 × 1.25 | 7.94 / 8.00 | 14.11 / 12.50 | pitch: 1.61 mm over ten threads |
| 5/16-24 UNF / M8 × 1 | 7.94 / 8.00 | 10.58 / 10.00 | pitch: 0.58 mm over ten threads |
| 3/8-16 UNC / M10 × 1.5 | 9.53 / 10.00 | 15.88 / 15.00 | diameter and pitch |
| 3/8-24 UNF / M10 × 1 | 9.53 / 10.00 | 10.58 / 10.00 | diameter and pitch |
| 1/2-13 UNC / M12 × 1.75 | 12.70 / 12.00 | 19.54 / 17.50 | diameter and pitch |
| 1/2-20 UNF / M12 × 1.25 | 12.70 / 12.00 | 12.70 / 12.50 | diameter: 0.70 mm apart |
| Error | What happens | How to check |
|---|---|---|
| A size table from a supplier lists #0-80 as coarse, gives 72 TPI as 0.363 mm or puts #3 at 40 TPI | #0 exists only as UNF 80; 72 TPI is 0.353 mm; #3 is 48 UNC or 56 UNF. A screw ordered from the wrong row starts and then binds. | Compare with the ASME B1.1 values in the inch table above. |
| A metric key in an inch socket | 5/32 in = 3.97 mm, 3/16 in = 4.76 mm, 5/16 in = 7.94 mm. A 5 mm key does not enter a 3/16 in socket; a 3/16 in key turns loose in a 5 mm socket and rounds it. | Read the key size from the table, not from the nearest metric key. |
| Ordering a “cup head” screw | Chinese catalogues translate the name of this screw literally as cup head; in the UK a cup head bolt is a coach bolt with a dome head and square neck. | Order by socket head cap screw, thread and length under the head. |
The key and spanner for every head are on the wrench size chart, and the full list of metric and inch look-alikes on the metric vs imperial thread chart.
Distributor Screw or Made to Order
Alloy steel cap screws in standard sizes are a distributor product, and we say so. We make the ones the catalogues do not carry, from 1 piece.
| What you need | In the catalogue? | What we make |
|---|---|---|
| Alloy steel cap screws, 12.9 metric fine or ASTM A574 inch, in a standard length | Yes — every fastener distributor | Buy it from stock; no one-off competes with a cold-formed, heat-treated cap screw. Standard parts: ask us too — we may have them in stock. |
| A length between the catalogue steps, or longer than the longest | No | Made to the length on the drawing, in 304, 316L or 42CrMo4. |
| A hollow cap screw: a bore along the axis for a cable, an air or oil line or a vent, plain or tapped | Not in most catalogues | Made to order: bore diameter and thread to the drawing. |
| An inch size, pitch or length the catalogue lists skip: UNEF, 8-UN, or a fine thread below #10 in 316L | Rarely | Made with the thread you name. |
| A4-80 / 316L for seawater, food or chemical plant | Partly | Made in 1.4404 where the A4 range stops. |
| A reduced or larger head, a drilled head for lock wire, a ground shank | No | Machined to the drawing; see drilled head bolts. |
| Left-hand or multi-start thread | No | Cut to the thread you measure. |
| Titanium grade 5 or 7075 aluminium to save weight | Partly | Made on request for low-preload joints; 6061 is too soft for a cap screw thread and we say so. |
Materials and Property Classes
| Material | Grade / head marking | Min. tensile / yield (MPa) | Where it is used |
|---|---|---|---|
| 304 stainless (1.4301) | A2-70; US catalogues: 18-8 | 700 / 450 | general corrosion resistance, food, outdoor |
| 316L stainless (1.4404) | A4-70 / A4-80 | 700 / 450; 800 / 600 | seawater, chlorides, chemical plant |
| Alloy steel, black oxide | 12.9 (metric, ISO 898-1); ASTM A574 (inch) | 1220 / 1100 (12.9) | machine building, high preload; the standard sizes come from distributors |
| Brass CW614N | — | — | electrical, non-magnetic, decorative |
| Titanium grade 5 | — | — | weight-critical assemblies |
| Aluminium 7075 | — | — | weight-critical, low preload; 6061 is too soft for the thread |
What to Send
The thread (metric size and pitch, or inch size and threads per inch), the length under the head, solid or hollow (with the bore size), the material or grade, the head form if it is not standard, and the quantity. For a replacement, a photo of the screw beside a rule and a ten-thread reading are enough. Quote in 12 hours.
Fine Thread and Inch Cap Screw FAQ
3/16 in (4.76 mm). The same key fits 1/4-28, because the head does not change with the pitch. Other common sizes: #10 takes 5/32 in, 5/16 takes 1/4 in, 3/8 takes 5/16 in, 7/16 and 1/2 take 3/8 in. A 5 mm metric key does not enter a 3/16 in socket, and a 3/16 in key turns loose in a 5 mm one.
Same diameters, different threads per inch: UNC is the coarse series and UNF the fine. A 1/4 in screw is 1/4-20 in UNC and 1/4-28 in UNF; a 3/8 in screw is 3/8-16 or 3/8-24. They do not interchange. Fine threads are used where the thread engagement is short, in thin walls and where an adjustment needs small steps. #0 exists only in UNF, as #0-80.
18-8 is the US catalogue name for the stainless family with about 18 % chromium and 8 % nickel. A 304 screw belongs to it and is grade A2 in ISO 3506-1; A2-70 on the head means that grade at 700 MPa minimum tensile strength and 450 MPa yield. For seawater and chlorides choose 316L, marked A4-70 or A4-80.
ISO 12474, for M8 to M36 in product grade A; it replaced ISO 21269. Before that the fine sizes were ordered as DIN 912 with a fine thread, and many catalogues still list them that way. The heads are the same as ISO 4762: an M10 × 1.25 screw has a 16 mm head, 10 mm high, with an 8 mm key.
Only if the tapped hole is re-cut, and usually it cannot be: 1/4-20 is 6.35 mm with a 1.27 mm pitch, M6 is 6.00 mm with 1.00 mm. On a US-built machine keep the inch screw. If the size or length is hard to find, we make the inch screw rather than change the hole.
Metric fine pitch from M7 to M20 in the pitches of the table, and larger sizes to a drawing; inch sizes from #0 to 1/2 in, in UNC and UNF, and above. Materials 304 (A2-70), 316L (A4-70 / A4-80), brass, titanium grade 5 and 7075; solid or hollow, with a standard or a modified head. Everything is made to order from 1 piece; send the thread, length under the head, material and quantity. Quote in 12 hours.