Because the teeth stay hidden under the fastener head, leaving the visible surface unmarked.

Toothed washers work by biting into the surfaces either side, and that bite leaves permanent marks.

With internal teeth, those marks are inside the head's footprint and covered by it.

That matters on trim, panels, enclosures and instrument faces, where a ring of gouges around a bolt head is unacceptable.

The compromise is that the teeth act at a smaller radius and therefore resist less rotation.

Where appearance does not matter, external teeth are the more effective choice.

Noticeably - locking torque depends on the radius at which the teeth act, and internal teeth act at the smallest radius available.

Resistance to rotation is a moment, so the same tooth force applied further from the centre produces more of it.

Internal teeth sit close to the bore by definition, so their leverage is minimal.

For a given nominal size the external tooth version of the same washer resists substantially more.

Neither type performs well in high-frequency transverse vibration, where prevailing-torque nuts and wedge washers are the proven answers.

Adequate preload in a properly designed joint remains more effective than either.

One whose tooth circle sits comfortably within the fastener head's bearing face.

If the teeth extend beyond the head, they mark the visible surface and the reason for choosing internal disappears.

If the washer is too small for the bolt, the teeth are crowded at the bore and deform rather than bite.

Head sizes vary between fastener types for the same thread, so the head dimension rather than the thread size should decide.

Pan and button heads are small; hex and flanged heads are larger and give more room.

Where the head is very small, a different locking method is usually more practical than a very small toothed washer.

Not really - the teeth take a permanent set when flattened and have blunted where they bit.

The twisting of each tooth as the joint tightens is a plastic deformation, not an elastic one.

A removed washer will show flattened teeth and bright marks where the edges engaged.

Refitting it gives much less bite, and the joint face is already marked, so the fresh washer has less to grip.

They are inexpensive, so replacement at every disassembly is the normal practice.

In documented maintenance regimes, locking components are commonly single-use regardless of appearance.

Poorly on hard surfaces and damagingly on plated ones - the mechanism needs something to bite into.

Against hardened steel the teeth cannot penetrate, so they flatten and the washer becomes a plain washer with a rough face.

On plated or painted surfaces the teeth break the coating, exposing the substrate at the wettest and most crevice-prone point of the joint.

On soft materials such as aluminium or plastic the teeth embed too far and the joint settles, losing preload.

The type suits medium-hardness materials where the teeth can bite without sinking.

Where the surfaces rule the washer out, a locking nut or a thread-locking adhesive is the alternative.

Countersunk-head fasteners, where a flat washer cannot bear properly against the tapered head.

A flat toothed washer under a countersunk head contacts on an edge and neither bites evenly nor bears evenly.

Conical or countersunk tooth washers are formed to match the head angle, so contact is across the tapered face.

The head angle differs between fastener standards, so the washer must match the fastener rather than just the size.

They sit within the countersink, which keeps the assembly flush.

Using a flat toothed washer with a countersunk screw is a common and ineffective improvisation.

External tooth washers are the usual choice for that; internal teeth can bond but reach much less surface.

Toothed washers are used to cut through paint and oxide to make a metal-to-metal connection for earthing and bonding.

External teeth act over a much larger circle, so they cut more surface and give a lower-resistance path.

Internal teeth are confined under the head, where the surface may already be clean or masked.

Bonding applications generally have their own specified washer type and material, often with a corrosion-resistant finish.

Where bonding is a safety function, the connection is tested rather than assumed from the hardware.