Transition washer: toothed or bimetallic?

A drawing, a bill of materials or a supplier quote says transition washer M12 — and two suppliers will ship you two physically different parts against that one line. One is a copper-alloy washer with a raised or serrated profile; the other is a two-sided copper–aluminium disc. They solve the same problem in different ways, they fail in different ways, and the words on the drawing usually do not tell you which one is meant. Here is how to tell them apart, which one belongs on which joint, and — if it is the bimetal disc — which of the eleven stocked sizes fits the lug you are holding.

In short

Two different parts are sold as transition washers: a toothed copper-alloy washer that pierces the aluminium oxide film as the bolt is tightened, and a roll-bonded copper–aluminium bimetal disc where the transition is already built into the material. The first makes the transition at assembly; the second was made at the mill. We supply the second one, in 11 stocked sizes from M6 to M20, all 2 mm thick.

2: parts sold under one name
M6–M20: 11 stocked sizes
2mm: Thickness
70/30: Al / Cu ratio

Two parts, one name

"Transition washer" is not one product — it is two, and which one you get depends on whose catalogue you are reading. Earthing and substation suppliers use the term for a profiled copper-alloy washer whose raised pattern bites through the aluminium oxide film when the bolt is tightened. Cable-lug and bimetal suppliers use exactly the same term for a roll-bonded copper–aluminium disc with a copper face and an aluminium face. Neither camp is wrong; each simply writes as though the other part did not exist.

Three related terms help, and one makes things worse. Bimetallic washer, Al/Cu washer and copper-aluminium transition washer all mean the two-sided disc. Bimetallic connector, though, is ambiguous: on some drawings it means a disc, on others a crimpable transition piece with an aluminium barrel and a copper palm — which is a cable lug, not a washer at all.

The two designs side by side

Toothed transition washer vs CUPAL bimetal washer

Same name, same bolted Al/Cu joint, two different mechanisms — and two different things that have to go right for the joint to last.

Aspect	Toothed transition washer	CUPAL bimetal washer
Material	Copper alloy throughout, sometimes tin-plated	Roll-bonded copper and aluminium — two distinct faces
How the transition is made	At assembly — the profile pierces the aluminium oxide film as the bolt is tightened	At the mill — cold roll-bonded and diffusion-welded through the whole disc
What its integrity depends on	Correct torque, once, and the protective grease or oil surviving	The material itself — no filler, no re-torque cycle
Which way round it goes in	As its maker's datasheet specifies — we publish nothing about a part we do not make	Copper face to whichever side is copper, aluminium face to whichever side is aluminium — the same washer, turned over
Do we supply it?	No	Yes — 11 stocked sizes, M6 to M20

AspectMaterial

Toothed transition washerCopper alloy throughout, sometimes tin-plated

CUPAL bimetal washerRoll-bonded copper and aluminium — two distinct faces

AspectHow the transition is made

Toothed transition washerAt assembly — the profile pierces the aluminium oxide film as the bolt is tightened

CUPAL bimetal washerAt the mill — cold roll-bonded and diffusion-welded through the whole disc

AspectWhat its integrity depends on

Toothed transition washerCorrect torque, once, and the protective grease or oil surviving

CUPAL bimetal washerThe material itself — no filler, no re-torque cycle

AspectWhich way round it goes in

Toothed transition washerAs its maker's datasheet specifies — we publish nothing about a part we do not make

CUPAL bimetal washerCopper face to whichever side is copper, aluminium face to whichever side is aluminium — the same washer, turned over

AspectDo we supply it?

Toothed transition washerNo

CUPAL bimetal washerYes — 11 stocked sizes, M6 to M20

The toothed washer: the transition is made at tightening

A toothed transition washer is copper alloy all the way through — it is not a bimetal. Its working face carries a raised or serrated profile, and when the bolt is tightened that profile is driven through the insulating oxide film on the aluminium surface, opening metal-to-metal contact points that would not exist otherwise. These washers are usually supplied greased or oiled, to keep moisture out of the freshly broken oxide layer, and often with backing washers.

That is a real engineering solution, and on the right joint it works. But it is worth being clear about what it rests on: a toothed washer's integrity is decided at installation. The profile has to be driven home correctly, once, and the protective grease has to survive in service. Get the tightening wrong and there is nothing in the part itself to fall back on.

That has one practical consequence. Torque figures for toothed washers are model-specific and published by their manufacturer — read that datasheet and use their numbers, not a generic bolt-torque table. We publish none, because we do not make that part.

The bimetal washer: the transition is made at the mill

The bimetal washer starts from a different premise. A Cu-ETP copper layer and an EN AW-1050A aluminium carrier are cold roll-bonded and diffusion-welded into a single composite, and the disc is cut from that material. Each face goes against its own metal — the copper face to the copper side of the joint, the aluminium face to the aluminium side — so like metal meets like metal at every contact face, and the critical Cu/Al interface is enclosed inside the disc, protected from moisture rather than sitting in the joint face.

That works in both directions. A copper lug on an aluminium busbar and an aluminium lug on a copper busbar are the same washer, fitted the other way round — the disc does not care which side of the joint the copper is on, and since it is 2 mm thick either way, the bolt length does not change.

Its integrity is built into the material, not into the fitting. There is no oxide film to pierce at assembly, no filler to dry out and no re-torque cycle to plan for — the transition was already made before the part left the mill. The article on galvanic corrosion prevention explains why the two metals must not touch directly in the first place.

Which one does your drawing mean?

Four checks, in the order that resolves it fastest:

Does the part have two visibly different faces — coppery on one side, silvery on the other? Then it is a bimetal disc.
Is it one colour throughout, with a raised or serrated pattern on the working face? Then it is toothed.
Does the specification mention grease, protective oil, backing washers, or a torque figure tied to a named model? That points to the toothed part — none of those apply to a bimetal disc.
Does the line say "bimetallic", "Al/Cu" or "Cu-Al"? That points to the disc. But if the drawing means a crimpable piece with an aluminium barrel and a copper palm, that is a cable lug — see our bimetal cable-lug range.

If the drawing still does not settle it, send us the part number or the drawing and we will tell you which of the two it is.

If it is the bimetal washer: choosing the size

We stock eleven sizes, M6 to M20, and several bolt sizes come in more than one outer diameter for larger lug palms.

Bolt size	Outer Ø	Max. lug cross-section	SKU
M6	16 mm	120 mm²	ICALCU6CS
M8	18 mm	120 mm²	ICALCU8CS
M10	23 mm	240 mm²	ICALCU10CS
M10 (large)	26 mm	300 mm²	ICALCU10CS2
M12	26 mm	240 mm²	ICALCU12CS2
M12 (large)	30 mm	400 mm²	ICALCU12CS
M12 (extra)	46 mm	500 mm²	ICALCU12CS3
M16	35 mm	500 mm²	ICALCU16CS
M16 (large)	46 mm	630 mm²	ICALCU16CS2
M20	37 mm	500 mm²	ICALCU20CS
M20 (large)	46 mm	630 mm²	ICALCU20CS2

The rule is simple: the bolt sets the bore (6.4 mm at M6, 21 mm at M20), and the lug sets the outer diameter. Take the row that covers the conductor cross-section marked on the lug palm; if the palm still overhangs the washer face, step up to the larger outer diameter — the washer has to cover the whole transition, not just the area around the bolt.

All 11 sizes are 2 mm thick, so stepping up the outer diameter never changes the bolt length you need.

Prices are on the product page: net list prices for all 11 CUPAL washer sizes, next to the quantity picker and the distributor cross-reference table. If you are working from another distributor's designation — a NEXANS Cupal M 10 Al-Cu, or a catalogue line that just says transition washer M12 — the part number cross-reference maps it onto the SKUs above by bolt size and nearest outer diameter.

What we supply, and what we do not

We supply the bimetal washer: 11 stocked sizes, 2–10 working days from order to delivery, no minimum order quantity, and an EN 10204 material certificate — 2.2 as standard, 3.1 on request. The difference between the two grades is explained in the article on the EN 10204 material certificate.

We do not supply the toothed oxide-piercing washer. If that is what your specification calls for, buy it from a maker of that part and follow their torque figures — a toothed washer is torque-critical in a way ours is not.

The cladding ratio: the number you can specify against

This is worth checking on any bimetal washer, ours included. Ours is 70 % aluminium / 30 % copper by layer thickness, density 4.6 g/cm³, conductivity about 41.9 m/Ω·mm². Not every supplier publishes a ratio, which makes it hard to hold one to a drawing. Parts on the market also differ substantially: some washers run as lean as 15 % copper, and some sheet is supplied at a 1:1 ratio, roughly 5.8 g/cm³. Neither is better in the abstract; it depends on whether the joint carries working current or is an earthing and bonding path where the bolt size sets the contact area. What matters is that the figure is published, so you can check it against your specification. If your specification calls for a particular ratio or a minimum copper thickness, tell us and we will confirm it before you order.

If neither size nor form fits

If your bolt pattern is not on the list, or the part is not a round disc at all, we cut discs, rectangles, strips and any profile you can draw, from your DXF, PDF or JPG in 0.5, 1.0, 1.5 and 2.0 mm. You get the quote within 24 hours and production takes 5–10 working days. The minimum is one full 500 × 500 mm board — a handful of one-off pieces still costs a board's worth of material. That route is on the cut-to-size page. Whether a laser or a punching tool suits your quantity is covered in laser cutting or punching for custom CUPAL parts.

And if the transition belongs at a cable end rather than in a bolted flat joint, a crimped bimetal lug is the better part — that and the other three ways of building a copper–aluminium transition are compared in CUPAL vs. transition methods.

Not sure which of the two your drawing means? Send us the part number or the drawing — we reply within 24 hours.

Frequently asked questions

Because two industries adopted the same name for different parts. Earthing and substation suppliers use it for a profiled copper-alloy washer whose raised pattern pierces the aluminium oxide film as the bolt is tightened. Cable-lug and bimetal suppliers use it for a roll-bonded copper–aluminium disc with a copper face and an aluminium face. The first makes the transition at assembly; the second has the transition built into the material. Check which mechanism a specification means before you order, because the two parts are not interchangeable on paper even though both fit the same bolt.

Look at the two faces. A bimetal washer is visibly two-tone — coppery on one side, silvery on the other — and smooth. A toothed washer is one colour throughout, usually a copper alloy and sometimes tin-plated, with a raised or serrated pattern on its working face, and it is frequently supplied greased. If the specification names a grease, a protective oil, backing washers or a model-specific torque figure, it is describing the toothed part.

No. We supply the roll-bonded copper–aluminium bimetal washer in 11 stocked sizes from M6 to M20, and we cut custom geometry from a drawing. A toothed oxide-piercing washer is a different part with a torque figure of its own, so buy it from the maker who publishes that figure.

Neither is better in the abstract — they differ in what has to go right. A toothed washer relies on the joint being tightened correctly once and on its protective grease surviving in service. A bimetal washer carries the transition inside the material, so it does not depend on the fitter and there is no filler to dry out. The same disc serves the mirror case, an aluminium lug on a copper busbar, simply turned over. If your specification already names one of the two, follow it; if it just says transition washer, decide on that difference.

We publish no torque figure for the washer itself, and none should be inferred. Tighten the joint to the torque given for the joint by the equipment or busbar manufacturer — the bimetal washer sits in that joint as a transition element and does not change its torque requirement. Toothed washers are different, because their profile has to be driven through the oxide film; their makers publish a torque figure per model.

Eleven stocked sizes from M6 to M20, with bores of 6.4 mm to 21 mm and outer diameters of 16 mm to 46 mm. Several bolt sizes come in more than one outer diameter for larger lug palms — M12, for example, in 26, 30 and 46 mm. Every size is 2 mm thick, so a wider washer still fits the same bolt length. Net list prices are published on the CUPAL washer product page.

No filler is required for the transition itself. The copper and aluminium layers are joined in the material and the Cu/Al interface is enclosed inside the disc, so there is nothing there for a compound to protect. Antioxidant paste is not forbidden — it is simply redundant, and one less step to get wrong. Do keep the contact faces clean and free of oil and grease, which impair conductivity, and follow the joint's own assembly instructions.

Send it to us and we will confirm the match. Our part number cross-reference page maps the codes we see most often — 880111, Cupal M 10 Al-Cu, ICALCU10CS — onto a stocked size, and the CUPAL washer product page carries a cross-reference table for the NEXANS Cupal M 8, M 10 and M 12 Al-Cu designations, mapped onto our SKUs by bolt size and nearest outer diameter. Check that outer diameter against your own lug footprint before ordering: a designation that matches on bolt size can still be too small for the palm. M16, M20 and the large variants are not in that table, so ask us for a quote on those.

Transition washer: toothed or bimetallic? How to tell which one your drawing means