Mating Screws Manufacturer in India | SEMS Screw & Washer Assemblies

Overview

SEMS Screw & Washer Assemblies (ISO 10644, DIN 6900): Manufacturer & Exporter

A mating screw comes with its washers already fitted. They cannot fall off, cannot be forgotten and cannot be put on in the wrong order, yet they turn freely under the head.

Exotic Alloy manufactures mating screws in Mumbai, India, for electrical panel, appliance, automotive, HVAC and machine builders. We supply across India and export to Morocco, North Africa, the Gulf and Europe. Each assembly is made in one run: the washer is fitted to the blank and the thread rolled behind it.

One part instead of two or three

On an assembly line a SEMS screw saves picking and placing washers, and nothing is lost into the product. For the screws themselves see hex cap screws and pan head screws; for loose washers see washers.

ISO 10644 DIN 6900 Captive washers Tapping versions A2 / A4

Mating screw at a glance

Also called: SEMS, combi screw, screw and washer assembly
Standards: ISO 10644, ISO 10673, DIN 6900, ISO 10510
Washers: Plain S / N / L, spring, tooth, conical
Captive: Washer bore smaller than the thread
Length: Under the head
Related: Washers, hex cap, pan head

M8 × 25 hex screw with a split spring washer and a plain washer, drawn to scale, washers in section and in plan. Thread shown schematically.

Specification

Mating Screw Specification

The range we manufacture and supply.

StandardsISO 10644 with ISO 10673 washers; DIN 6900 with DIN 6902, 6905, 6907, 6908 washers; ISO 10510 tapping

Size rangeM2–M12; tapping ST2.2–ST9.5

Screw headsHexagon, pan, cheese; cross recess, hexalobular or slot

Plain washersSmall (S), normal (N), large (L)

Lock washersSplit spring, serrated (tooth), conical spring; spring + plain

LengthsUnder the head; standard lengths to 100 mm, longer to order

MaterialsSteel 4.8 / 8.8; stainless A2 / A4

FinishZinc plated, zinc flake; stainless plain or passivated

CertificationMill certificate; EN 10204 3.1; RoHS on request

PackingBoxes of 100 to 10,000; bulk for feeders

Washer options

Plain, Spring, Tooth & Conical Washer Combinations

The washer decides what the joint gets: bearing area, spring or bite.

Six M6 combinations at the same scale. The large plain washer spreads the load; spring, tooth and conical washers add spring or bite.

Combination	Standard	What it adds	Typical use
Plain washer S / N / L	ISO 10644, DIN 6900 + DIN 6902	Bearing area, covers the hole	Sheet metal, plastics, slotted holes
Split spring washer	DIN 6905 (withdrawn)	Some spring in soft joints	General assembly, legacy designs
Spring + plain washer	DIN 6905 + DIN 6902 / ISO 10673	Spring plus bearing area	Panels, brackets (as in the photo)
Serrated (tooth) washer	DIN 6907 (withdrawn)	Teeth bite into the surface	Electrical contact through coatings
Conical spring washer	DIN 6908	Flat-spring force, large area	Joints that settle, soft gaskets

Dimensions

Mating Screw Washer Dimensions

ISO 10673 plain washers from M2 to M12 at the same scale, small, normal and large series around the captive bore.

Size	Bore d₁ min / max	Small S d₂ × h	Normal N d₂ × h	Large L d₂ × h	a max 2P
M2	1.75 / 1.85	4.5 × 0.6	5 × 0.6	6 × 0.6	0.8
M2.5	2.25 / 2.35	5 × 0.6	6 × 0.6	8 × 0.6	0.9
M3	2.75 / 2.85	6 × 0.6	7 × 0.6	9 × 0.8	1
M3.5	3.2 / 3.32	7 × 0.8	8 × 0.8	11 × 0.8	1.2
M4	3.6 / 3.72	8 × 0.8	9 × 0.8	12 × 1	1.4
M5	4.55 / 4.67	9 × 1	10 × 1	15 × 1	1.6
M6	5.5 / 5.62	11 × 1.6	12 × 1.6	18 × 1.6	2
M8	7.4 / 7.55	15 × 1.6	16 × 1.6	24 × 2	2.5
M10	9.3 / 9.52	18 × 2	20 × 2	30 × 2.5	3
M12	11 / 11.27	20 × 2	24 × 2.5	37 × 3	3.5

ISO 10644 / ISO 10673 (2009), mm: d₂ maximum, h nominal. a = distance from the washer to the first full thread.

Size	Spring DIN 6905 d₂ max × s, free h	Tooth DIN 6907 d₂ × s, teeth	Conical DIN 6908 d₂ × s, h	Bore d₁ min all
M2.5	4.55 × 0.6, 0.9	5.5 × 0.4, 9	6 × 0.5, 0.61–0.72	2.25
M3	5.6 × 0.7, 1.1	6 × 0.4, 9	7 × 0.6, 0.72–0.85	2.7
M3.5	6.1 × 0.7, 1.1	7 × 0.5, 10	8 × 0.8, 0.92–1.06	3.2
M4	6.95 × 0.8, 1.2	8 × 0.5, 11	9 × 1, 1.12–1.3	3.6
M5	8.55 × 1, 1.5	9 × 0.6, 11	11 × 1.2, 1.35–1.55	4.55
M6	11 × 1.3, 2	11 × 0.7, 12	14 × 1.5, 2.3–2.6	5.5
M8	13.95 × 1.6, 2.45	14 × 0.8, 14	18 × 2, 2.8–3.1	7.4
M10	16.95 × 1.8, 2.85	18 × 0.9, 16	23 × 2.5, 3.3–3.6	9.3
M12	19.7 × 2.1, 3.35	–	29 × 3, 3.8–4.1	11

DIN 6905 (1990, withdrawn), DIN 6907 (1990, withdrawn, to M10) and DIN 6908 (1995), mm. s = section thickness, h = free height.

Size	a max	Normal d₂ × h	Large d₂ × h
ST2.2	0.8	5 × 1	7 × 1
ST2.9	1.1	7 × 1	9 × 1
ST3.5	1.3	8 × 1	11 × 1
ST4.2	1.4	9 × 1	12 × 1
ST4.8	1.6	10 × 1	15 × 1.6
ST5.5	1.8	12 × 1.6	15 × 1.6
ST6.3	1.8	14 × 1.6	18 × 1.6
ST8	2.1	16 × 1.6	24 × 2
ST9.5	2.1	20 × 1.6	30 × 2.5

ISO 10510:2011 tapping screw and washer assemblies with plain washers, mm.

How it works

How the Washer Is Made Captive

The thread is rolled after the washer is fitted. Rolling raises the outside diameter above the washer bore, so the washer is trapped under the head.

The washer bore is deliberately smaller than the thread: 7.4 to 7.55 mm for M8, 4.55 to 4.67 mm for M5. It slides onto the plain, unthreaded blank; then the thread is rolled, and the rolled thread is too big for the washer to pass.

That is why a SEMS washer cannot be fitted afterwards, and why the first full thread is close to the head: ISO 10644 limits it to 2 pitches (a max = 2P).

Benefit	Why
Faster assembly	One part to pick and drive
No missing washers	The washer cannot be left out
Right order every time	Spring and plain washer pre-stacked
Nothing lost in the product	No loose washers in electronics

DIN vs ISO

DIN 6900 / DIN 6902 vs ISO 10644 / ISO 10673

The plain washers have the same bore and the same outside diameters in DIN and ISO, so they look alike. But several ISO washers are thinner, which changes the stack height and the screw length you need. The differences are shown in bold.

Size	Small ISO S / DIN C, h	Normal ISO N / DIN A, h	Large ISO L / DIN B, h	Same d₂?	DIN 6902 mass A / B / C, kg per 1,000
M2.5	0.6 / 0.6	0.6 / 0.6	0.6 / 0.8	yes	0.11 / 0.29 / 0.07
M3	0.6 / 0.6	0.6 / 0.6	0.8 / 0.8	yes	0.15 / 0.36 / 0.11
M3.5	0.8 / 0.8	0.8 / 0.8	0.8 / 0.8	yes	0.27 / 0.55 / 0.19
M4	0.8 / 0.8	0.8 / 0.8	1 / 1	yes	0.34 / 0.81 / 0.25
M5	1 / 1	1 / 1	1 / 1.6	yes	0.49 / 2.01 / 0.37
M6	1.6 / 1.6	1.6 / 1.6	1.6 / 1.6	yes	1.12 / 2.9 / 0.89
M8	1.6 / 2	1.6 / 2	2 / 2	yes	2.48 / 6.43 / 2.1
M10	2 / 2.5	2 / 2.5	2.5 / 2.5	yes	4.83 / 12.54 / 3.66
M12	2 / 3	2.5 / 3	3 / 3	yes	8.41 / 23.08 / 5.16

Washer thickness h, mm: ISO 10673 series S / N / L against DIN 6902 forms C / A / B (DIN 6900-1). Bold = different.

Length finder

Mating Screw Length Finder

Pick the size and washers, enter the part thickness, and choose a nut or a tapped hole to get the screw length.

Screw & joint

Size (hex head)

Washers

Part thickness, mm

Into

Thread engagement in a tapped hole, mm

Our calculation: length ≥ part + washer stack (tightened: plain h, flattened spring / tooth / conical s) + nut height m (ISO 4032 max) + 2 pitches, or + your thread engagement in a tapped hole; then the next standard length. Washer data ISO 10673 and DIN 6905 / 6907 / 6908.

Result

25

mm standard length

3.2

mm washer stack

Minimum length:22.5 mm

Bearing diameter:16 mm

Head s × k:13 × 5.3 mm

Designation:M8 × 25

Materials

Mating Screw Materials & Finishes

Screw	Washers	Finish	Best for
Steel 4.8	Steel	Zinc plated	Light assembly, panels
Steel 8.8	Hardened steel	Zinc plated, zinc flake	Machinery, automotive
Stainless A2	Stainless	Plain, passivated	Electrical, food, outdoor (as in the photo)
Stainless A4	Stainless	Plain, passivated	Marine and chemical plant

Washer hardness is matched to the screw’s property class so the washer does not sink under the head.

Steel 4.8

Low carbon

Zinc plated.

1018 1022

Steel 8.8

Quenched & tempered

Higher clamp loads.

1035 1045

Stainless A2

304

As in the photo.

304

Stainless A4

316

Chlorides and washdown.

316

Manufacturing

How Our Mating Screws Are Made

1

Wire

Steel or stainless wire, certified by heat.

2

Heading

Head and plain blank formed.

3

Washer feeding

Washers fed and fitted on the blank.

4

Thread rolling

The rolled thread traps the washers.

5

Heat treatment

8.8 screws quenched and tempered.

6

Finishing

Plating or passivation as an assembly.

Applications

Where Our Mating Screws Are Used

Electrical panels

Terminals, busbars and earth points.

Automotive

Brackets, harness clips and trim.

Appliances

Fast assembly on production lines.

HVAC

Panels, fans and housings.

Solar & outdoor

Stainless combis on frames and boxes.

Machinery

Guards, covers and brackets.

Quality & testing

Mating Screw Inspection & Certification

Check	Method	Applied to
Washer present & order	Visual / automatic sorting	Every lot
Washer turns freely	Manual spin check	Every lot
Washer d₂, h	Callipers to ISO 10673 / DIN	Every lot
Thread	Ring gauges	Every lot
Hardness	Screw and washer	Heat-treated lots
Material	PMI / chemical analysis	Stainless lots

Every shipment includes

Mill certificate with heat number
Dimensional report
Lot number on every box

On request

EN 10204 3.1 certificate
Salt spray test for plated lots
RoHS declaration

FAQ

Mating Screw Questions, Answered

Mating screws, also called SEMS or combi screws, are screws supplied with one or two washers already fitted. The washer goes onto the screw blank before the thread is rolled, so it is captive but free to turn. ISO 10644 covers assemblies with plain washers from M2 to M12; the DIN 6900 series and its washers DIN 6902, 6905, 6907 and 6908 cover plain, spring, serrated and conical washers.

Its bore is smaller than the thread. For M8 the washer bore is 7.4 to 7.55 mm against an 8 mm thread: the washer is fitted on the plain blank, and rolling the thread raises the outside diameter above the bore, trapping the washer under the head.

Small, normal and large outside diameters of the plain washer. For M6 they are 11, 12 and 18 mm in ISO 10673. Use the large series on soft materials, slotted holes or oversized holes, and the small series where space is tight.

Plain washers in small, normal or large series; split spring washers (DIN 6905); serrated (tooth) lock washers (DIN 6907); conical spring washers (DIN 6908); and combinations such as a split spring washer plus a plain washer, as in our photo.

Not reliably in highly preloaded joints. Split spring washers are no longer regarded as an effective locking device for such joints, and several DIN spring washer standards, including DIN 6905, have been withdrawn. They still help keep some clamp in soft or settling joints; for vibration, use a locking method designed for it.

The plain washers have the same bore and outside diameters, but some ISO washers are thinner: for M8 the normal washer is 1.6 mm thick in ISO 10673 against 2 mm in DIN 6902, and for M12 2.5 mm against 3 mm. Check old drawings that call up DIN numbers.

Under the head, as for the screw on its own; the washers sit within that length. Allow for the washer stack when choosing the length: our finder adds the washers, the part and the nut or thread engagement for you.

Steel screws in property classes 4.8 and 8.8, usually zinc plated, with hardened steel washers; and A2 or A4 stainless steel screws with stainless washers, like the ones in our photo.