Washer specification: “an M4 washer” is three different parts

Three outer-diameter series, one nominal size

ISO 7091 is the normal series at a coarser product grade, which is why the same nominal washer can also differ in tolerance and finish quality without differing in dimensions on paper.

How to read a washer specification

A washer is specified by five things, and the first three are dimensions:

• Inside diameter — and note it is larger than the screw, because the screw has to pass through. An “M4 washer” has an inside diameter above 4 mm, not 4 mm
• Outside diameter — the series above
• Thickness — which at small sizes is comparable to the head height, so it is not free
• Material — carbon steel, stainless, brass, nylon, fibre
• Hardness class and finish — the one most often omitted, and the one that matters most

So add a washer — and what it is actually doing is not that

“It spreads the load” is the usual answer and it is the least important of the four, except in soft materials. In order of how often it is the real reason:

• It gives a consistent bearing surface. The underhead friction coefficient is most of what decides how much of your torque becomes clamp force, and a washer makes that surface the same every time regardless of what the part surface is like — which is the whole problem with torque control
• It protects the surface from being scored by the rotating head — cosmetic on a painted part, functional on a sealing face
• It bridges an oversized or slotted hole, which is the case where the large series exists for a reason
• It spreads the bearing pressure — genuinely important in aluminium, plastic and wood, and largely irrelevant in steel

Adding a washer is a joint change

Put a washer under a head that previously bore directly on the part and the friction interface moves. It is now screw-to-washer and washer-to-part rather than screw-to-part, with different coefficients and a second interface that can slip.

• The clamp force from the same torque changes, usually upward with a smooth hard washer — which can overload a thread that was previously fine
• The grip length increases by the washer thickness, which is slightly helpful against embedding and slightly reduces thread engagement if the screw length is unchanged
• A free washer can rotate with the head, moving the sliding interface to the washer-to-part face without warning

The types, and whether they do what they claim

Below M3, three things change

• Washer thickness becomes a significant fraction of the head height — and so does everything else at these sizes, so it visibly changes how much thread is left engaged
• Handling cost rises sharply. Feeding a loose M2 washer reliably is harder than feeding the screw, which is exactly why SEMS screws exist — the washer is assembled at the factory and cannot fall off
• Flatness and burr matter proportionally more. A burr that is negligible under an M8 head is a tilt under an M2 one

What to put on the drawing

• The standard number — ISO 7089, ISO 7092 or ISO 7093, not “M4 washer”
• Material and hardness class, because a soft washer is an embedding source
• Finish, and check it against the parts it touches — a washer is a third metal in the joint
• Whether it is required at all, stated as a function. “Washer to provide a consistent bearing surface” survives a cost-down review; an unexplained line item does not

This page covers step 3, the head. The whole order is substrate, thread, head, drive, finish, documentation, and why doing it out of order is rework rather than a tweak is in specifying a screw.

Common questions

Which way round does a flat washer go?

For an ordinary plain washer the standards do not say, because there is nothing to orient. Neither ISO 7089 nor ISO 7090 specifies an assembly orientation. The common advice to put the chamfered side toward the bolt head does not apply to ISO 7090 either, because its chamfer is on the outside diameter — its scope says so in those words — and its bore is the same as ISO 7089, so turning it round provides no clearance for the underhead fillet. A washer that genuinely clears the fillet needs a countersunk bore, and the rule that exists for it is in NASA-STD-5020A: countersunk face toward the bolt head. What does matter on an ordinary stamped washer is flatness and burr, and that matters proportionally more at small sizes.

How do you read a washer specification?

By five things: inside diameter, outside diameter, thickness, material, and hardness class with finish. The inside diameter is larger than the screw, since the screw must pass through — an M4 washer has an inside diameter above 4 mm. The outside diameter is set by the series, and hardness is the item most often omitted and the one that matters most, because a washer softer than the surrounding parts deforms under the head and loses preload.

Why do washers of the same size have different outer diameters?

Because plain washers exist in several outer-diameter series. Smallest to largest: small (ISO 7092 / DIN 433), normal (ISO 7089 / DIN 125), large (ISO 7093 / DIN 9021, roughly three times the nominal diameter) and extra large (ISO 7094). Between small and large the outer diameter more than doubles, which decides whether the washer clears a counterbore and how much bearing area a soft material gets. Specify the standard number rather than the size.

What is a washer actually for?

Most often, to give a consistent bearing surface so the underhead friction is repeatable — and underhead friction is most of what decides how much of the applied torque becomes clamp force. Protecting the surface from the rotating head and bridging an oversized or slotted hole come next. Spreading the bearing pressure, the usual textbook answer, genuinely matters in aluminium, plastic and wood and is largely irrelevant in steel.

Does adding a washer change the joint?

Yes. The friction interface moves from screw-to-part to screw-to-washer plus washer-to-part, with different coefficients and a second surface that can slip, so the clamp force obtained from the same torque changes — usually upward with a smooth hard washer, which can overload a thread that was previously fine. Grip length also increases by the washer thickness. Treat it as a change requiring re-verification, not as a free bill-of-materials addition.

Do spring lock washers work?

NASA’s fastener design manual assesses the split type as useless for locking once flattened by preload, and it flattens well below normal clamp force. Toothed washers work by biting into the mating face, which removes the coating and can start a corrosion problem. Wedge-lock ramped pairs are effective under transverse-vibration testing. A disc spring solves a different problem again — it maintains preload through settling and thermal cycling rather than preventing rotation.

References

• ISO 7089 — Plain washers — Normal series, product grade A
• ISO 7092 — Plain washers — Small series, product grade A
• ISO 7093-1 — Plain washers — Large series
• NASA RP-1228 — Fastener Design Manual (assessment of split-type lockwashers)