A SEMS screw is a screw with its washer already on it: the thread is rolled after the washer is fitted, so the washer is captive for life. Three-piece assemblies carry a spring washer under a flat washer; two-piece carry one or the other. The series pairs a fixed washer with each head and size; a washer of another diameter, another material or another function is a SEMS that has to be built.
EKINSUN LTD is a custom parts manufacturer that builds SEMS assemblies to drawing when the stock 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 stock table below is read from a 2026 catalogue; the washer table explains what each washer adds; the negative-space table marks where the series stops.
"M4 pan Torx with a 12 mm flat washer captive, stainless, 2,000 pieces. The three-piece has a 9 mm washer and a spring washer we do not want."
A SEMS is chosen from the washer outward; the screw is the easy part.
| Assembly | Washers | Stock range | Stock 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.
| 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, 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 |
|---|---|---|
| Pan, hex, socket or Torx three-piece SEMS M1.6–M12 in A2 or zinc steel | Yes | Buy it from stock. No one-off competes with a rolled SEMS assembly. |
| Two-piece SEMS with spring, flat or square washer in the stock ranges | Yes | Stocked; buy it. |
| A flat washer of another outside diameter or thickness, captive on a stock screw | No | Assembled and rolled to your washer, from 1 piece; the washer stamped or turned first if it is not stock. |
| 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. |
| Material | Designation | Where it fits here |
|---|---|---|
| Stainless 304 | 1.4301 / A2 | the stock 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 | stock steel SEMS M1.6–M12 |
| Alloy steel 12.9 | black or nickel | stock socket three-piece SEMS M3–M12 |
| Brass | CW614N / CuZn37 | stock 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 |
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 stock or turned screw and the thread rolled or cut after it, so the washer is captive; from 1 piece.
EKINSUN LTD 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 (stock or made), the screw (stock or turned), the assembly step and quantity. A stock screw with a stock 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.
Washer type, outside diameter and material; then thread, head, drive, length, screw material and quantity. From 1 piece, quote within 12 hours.
SEMS assemblies built to your washer, head, thread and material — from 1 piece, quote in 12 hours.