EKINSUN is a custom parts manufacturer that builds SEMS assemblies to drawing when the standard pairing does not exist: a large-OD washer on a small screw, a sealing washer under a hex head, a serrated washer in stainless, a PEEK or nylon washer on a brass screw, a 316L assembly. The size ranges are those a distributor stocked in 2026.
A typical request: M4 pan Torx with a 12 mm flat washer captive, stainless, 2,000 pieces, where the stock three-piece has a 9 mm washer and an unwanted spring washer.
A SEMS is chosen from the washer outward; the screw is the easy part.
SEMS types and size ranges
| Assembly | Washers | Size range | Common materials | Notes |
|---|---|---|---|---|
| Pan Phillips three-piece | spring + flat | M1.6–M8 | A2; zinc, yellow-zinc, blue-zinc, black, nickel steel; brass M2.5–M8 | the standard electrical and appliance SEMS |
| Hex three-piece | spring + flat | M3–M12 | A2; zinc steel | hex head for spanner driving |
| Hex Phillips (indented) three-piece | spring + flat | M3–M10 | A2; zinc steel | either driver |
| Socket cap three-piece | spring + flat | M2–M12 | A2; 12.9 nickel and black | knurled or plain head; machine building |
| Button socket three-piece | spring + flat | M2–M8 | A2 | low domed head |
| Torx pan three-piece | spring + flat | M3–M6 | A2 | power-driven lines |
| Truss two-piece | spring washer | M2 | A2 | wide head on thin sheet |
| Pan two-piece | spring washer | M2.5–M5 | A2; zinc steel | lighter assembly |
| Pan / truss two-piece with flat washer | flat washer | M2–M8 | A2; zinc steel | load spreading only |
| Square-washer two-piece | square washer | M3–M5 | zinc steel | terminals, wire clamps |
| Socket three-piece with plain shank | spring + flat | M2.5–M6 | A2 | unthreaded section under the head |
| Patch-locked SEMS | spring + flat, nylon patch | M2–M8 | A2 | prevailing torque added |
DIN 6900 to DIN 6905 cover the German combination-screw forms; ISO 10644 covers screw and washer assemblies with plain washers, with the washers themselves in ISO 10673.
Need SEMS screws in a size, thread or material no catalogue lists? EKINSUN is a custom parts manufacturer: we make SEMS screws to your drawing, sketch or sample, steel or 304 stainless steel, from 1 piece, quoted in 12 hours and shipped duties paid. Standard sizes can often be supplied from stock; the more pieces, the lower the unit price. All custom fasteners
What each washer in a SEMS does
| Washer | Adds | Leave it out when |
|---|---|---|
| Flat washer | bearing area; protects paint and soft material; bridges a slot | the head already has a flange |
| Spring (split) washer | some take-up of settling; a habit more than a lock under high preload | the joint is torqued to a value and a real lock is wanted |
| Serrated or tooth washer | bite into the mating face; electrical contact through paint | the face is soft aluminium 6061 or plastic |
| Square washer | a flat that cannot rotate under a terminal | — |
| Sealing washer (EPDM-bonded) | a seal under the head | the joint is dry |
| Nylon or PEEK washer | insulation and a soft face | the joint is hot or torqued hard |
| Large-OD washer | load spread over thin or slotted sheet | the head is already wide |
Read the pitch before you order the SEMS 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, the part is a standard size: buy it off the shelf, or have it made to order from 1 piece. If any of the three falls outside the table, we make it to your reading. The full measuring routine, including the inch traps
Where the SEMS catalogue stops
| What you need | In the catalogue? | What we make |
|---|---|---|
| Pan, hex, socket or Torx three-piece SEMS M1.6–M12 in A2 or zinc steel | Yes | Buy it from stock, or have it made to order from 1 piece. No one-off competes with a rolled SEMS assembly. Standard parts: ask us too, we may have them in stock. |
| Two-piece SEMS with spring, flat or square washer in the size ranges | Yes | Standard sizes: buy them off the shelf, or have them made to order from 1 piece. Standard parts: ask us too, we may have them in stock. |
| A flat washer of another outside diameter or thickness, captive on a standard screw | No | Assembled and rolled to your washer, from 1 piece; the washer stamped or turned first if it is not standard. |
| A sealing, serrated, nylon, PEEK or copper washer captive on a screw | No | Assembled; the washer material chosen for the joint. |
| A SEMS in 316L, brass above M8, or titanium | No | Screw turned and washer matched in the same alloy. |
| A SEMS with a head or drive the series does not pair: cheese head, hex socket on a truss, pin-Torx | No | Screw turned with the recess machined; washer assembled. |
| A SEMS in a fine, left-hand or inch thread, or above M12 | No | Thread cut to your reading after the washer is fitted. |
| A SEMS in aluminium 6061 or 7075 | Partly | Anodised 7075 screws with captive aluminium washers on request for light, decorative joints; not for preload, and we say so. |
Materials
| Material | Designation | Where it fits here |
|---|---|---|
| Stainless 304 | 1.4301 / A2 | the standard SEMS in every head; washers in 304 or 301 spring |
| Stainless 316L | 1.4404 / A4 | built to order for marine and chemical joints |
| Free-machining stainless | 303 / 1.4305 | custom turned SEMS screws |
| Carbon steel | zinc, yellow-zinc, blue-zinc, black, nickel | standard steel SEMS M1.6–M12 |
| Alloy steel 12.9 | black or nickel | standard socket three-piece SEMS M3–M12 |
| Brass | CW614N / CuZn37 | standard M2.5–M8; electrical |
| Aluminium 6061 | AlMg1SiCu | the panel material a serrated washer must not bite |
| Aluminium 7075 | AlZn5.5MgCu | anodised light SEMS on request |
| Nylon 66, PEEK, EPDM, copper | washer materials | insulating, sealing and soft-face washers built into custom SEMS |
Pre-applied threadlocker and large-run assemblies
A SEMS screw can arrive with its locking already on the thread: a pre-applied threadlocker patch, a dry adhesive coating on the first threads that stays inert in the box and cures when the screw is driven and the two threads crush the coating together. It replaces the bottle of liquid threadlocker on the assembly line and makes the locking a property of the part, so a distributor or an OEM can buy “M3 × 8 A2 with washer and blue patch” as one item. Colour is not a strength code on pre-applied coatings: the high-strength precote 80 is red and other colours are made to order, so the strength is specified by product or breakaway torque, not by colour. A patch is single-use for most grades, so a screw that will be removed and refitted many times needs a nylon patch or a mechanical lock instead.
| Item | What to specify | How we build it |
|---|---|---|
| Screw | Thread, length, head form and drive, property class or stainless grade (A2 / A4), and whether the head must match a sample or a standard, the two can differ | Standard sizes from stock or made to drawing from 1 piece; for a run, the head form is confirmed on a sample before coating |
| Washer | Outside and inside diameter, thickness, material and finish; captive (SEMS) or loose | Pre-assembled under the head before the patch is applied, so the coating never reaches the washer |
| Patch | Strength (medium, serviceable, or high, permanent), colour, start position and length along the thread (for example threads 2 to 5), and the breakaway or prevailing torque the assembly must reach | Applied by a coating partner to the stated threads; cure on assembly; breakaway torque checked on a sample from each lot |
| Run size | Pieces per lot and per year; packing (bulk, bagged by 100 or 500, tray) | Tens of thousands of sets per lot; the more pieces, the lower the unit price; a sample lot first, then the run from the approved sample |
| Paperwork | Material certificate, plating certificate, dimensional report, torque report | EN 10204 3.1 for the screw material; inspection report with the shipment |
If the drawing says DIN 912 but the sample head is lower, wider or has a different socket, say which one rules before the run: a patch-coated screw cannot be reworked.
Washer sizes for DIN 912 M3 to M8, why a 3.2 mm washer is not captive on M3, and which coating suits small A2 threads: DIN 912 screws with pre-applied threadlocker and washer.
Related pages
- fastener types chart, all families with standard and size range
- washer types, every washer a SEMS can carry
- machine screws, head and drive types chart
- pointed, flat and drill-point ends, tapping threads with captive washers
- socket, button and countersunk heads, the socket three-piece parent
- serrated and plain flange nuts, the nut with its washer built in
- Custom washers, washers machined to size
Frequently asked questions
A screw with one or two washers captive under the head: the washers are fitted before the thread is rolled, so the thread's outside diameter is larger than the washer hole and the washer cannot come off. SEMS stands for pre-assembled screw and washer; combination screw is the same thing.
Two-piece has one captive washer, usually a spring washer, a flat washer or a square terminal washer. Three-piece has a spring washer and a flat washer stacked: the spring takes up settling and the flat spreads the load and protects the surface.
Only a little. Under a properly preloaded screw a split spring washer is flat and adds no locking; it helps take up settling in soft joints. Where a real lock is needed, specify a serrated washer, a patch-locked SEMS or a prevailing-torque nut.
Yes. A large-OD, sealing, serrated, copper, nylon or PEEK washer is fitted to a standard or turned screw and the thread rolled or cut after it, so the washer is captive; from 1 piece.
EKINSUN is a custom parts manufacturer that builds SEMS assemblies to drawing in 304 and 316L stainless, steel, brass and titanium with any captive washer, head and thread, from 1 piece, with a quote within 12 hours.
No list price; the drivers are the washer (standard or made), the screw (standard or turned), the assembly step and quantity. A standard screw with a standard large-OD washer made captive is close to catalogue money; a 316L Torx screw with a bonded sealing washer is priced by its parts. The quote separates them.
Send the washer first, then the screw
Washer type, outside diameter and material; then thread, head, drive, length, screw material and quantity. From 1 piece, quote within 12 hours.