The Bench · N° 22

What an overcoil does to a hairspring

An overcoil lifts and curves the hairspring's outer turn so the spring can breathe more concentrically. It can improve positional behaviour, but it is not the only route to a good oscillator.

Hen & Mills Editorial · 4 min read · 29 August 2026

A hairspring should expand and contract around its centre as evenly as possible. In a flat spiral, the outer attachment can pull the spring slightly to one side as it breathes.

An overcoil changes the path of the outer turn. It rises above the main spiral, curves back towards the centre and reaches the stud from a more favourable direction.

The aim is better concentric breathing and more consistent timing in different positions.

Why the outer attachment matters

The hairspring is fixed at its inner end to the collet and at its outer end to the stud. Those attachments influence how the spiral changes shape during each oscillation.

If the spring's centre of gravity moves sideways as it expands and contracts, gravity can affect the oscillator differently when the watch changes position.

The terminal curve is designed to reduce that movement. It cannot remove every source of positional error, but it can improve the spring's behaviour.

The shape of an overcoil

Near its outer end, the spring is bent upward out of the flat plane. The raised section then follows a calculated curve over the main coils before reaching the stud.

It is often associated with the Breguet overcoil, although terminal-curve theory and practice include several forms.

The important point is not simply that one coil sits higher. Its horizontal and vertical shapes must work together with the balance, collet and attachment position.

How it helps the spring breathe

A well-formed overcoil lets the main spiral expand and contract more concentrically. This can keep the spring's centre of gravity closer to the balance axis during oscillation.

That reduces one mechanism by which gravity produces rate differences between vertical positions.

The benefit is most valuable when the rest of the oscillator is also well made and adjusted. Poor poise, damaged pivots or an incorrectly formed spring can overwhelm it.

Why the spring needs space

Lifting the terminal curve makes the assembly taller. The overcoil must clear the balance bridge, regulator components and coils below it throughout its motion.

That can influence movement thickness and bridge design. It also creates additional surfaces that must not touch after a shock or during service.

A flat hairspring is easier to package in a thin movement because its coils remain in one plane.

Forming and adjusting it

Traditional overcoils require skilled forming. The curve must be shaped without twisting, kinking or weakening the spring.

Small changes can alter centring, flatness and beat. During service, a watchmaker needs suitable magnification, tools and experience to correct the terminal curve if it has been disturbed.

Modern manufacturing can produce highly repeatable springs, but final oscillator performance still depends on assembly and adjustment.

The role of the regulator

An overcoil can be used with an indexed regulator or a free-sprung balance. The two features solve related but distinct problems.

The terminal curve addresses how the hairspring breathes and attaches. A regulator changes its effective active length, while a free-sprung system adjusts balance inertia.

It is therefore incorrect to treat an overcoil as proof that a movement must use one particular regulating architecture.

Why many good watches use flat springs

A carefully designed flat hairspring can perform very well. Improved materials, precise manufacturing, optimised attachment geometry and detailed adjustment have reduced the overcoil's historical advantage in some designs.

Flat springs also suit thin movements and automated assembly. Silicon hairsprings can be manufactured with complex terminal geometry in their own plane, allowing designers to influence concentricity without a traditional hand-raised coil.

The result should be judged by performance, not by whether the spring visibly climbs above itself.

Materials change the options

Steel-alloy hairsprings can be formed and adjusted using established workshop techniques. Silicon behaves differently and is produced through microfabrication rather than bent into shape at the bench.

Silicon can integrate terminal features with high repeatability and offers resistance to magnetism. It can also require replacement rather than traditional reshaping if damaged.

Neither material makes geometry irrelevant. It changes how that geometry is created and serviced.

What an overcoil cannot guarantee

The feature does not guarantee chronometer performance, immunity to shocks or equal rates in every position. Balance poise, escapement condition, lubrication, amplitude and regulation all remain important.

An overcoil that touches another component or has been badly adjusted may perform worse than a sound flat spring.

Marketing close-ups show a recognisable feature. Timing results show whether the complete oscillator is doing its job.

What to look for in practice

If you can view the movement, an overcoil will appear as a raised outer section crossing above the flat spiral. It should have clear space around it and should not touch neighbouring coils.

Owners should not try to reshape it. Even a slight accidental bend can alter the watch's rate and positional behaviour.

During service, ask whether the watchmaker is equipped to work on that type of hairspring, especially in a vintage or unusual calibre.

The useful way to understand it

The overcoil is a geometric solution to a specific problem: the outer attachment of a flat spiral can disturb concentric breathing.

By lifting and redirecting the terminal curve, the designer can reduce that disturbance and improve positional consistency. The cost is height, manufacturing complexity and more demanding adjustment.

It remains an elegant solution, but it is one solution among several. A well-executed flat spring is not automatically inferior, and a visible overcoil is not a substitute for measured performance. Measured positional results remain the fair comparison.