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The Encyclopedia of Horology

Component record · Regulation

Overcoil Terminal Curve

Partially verified

A hairspring whose outer coil is raised and curved over the body of the spring.

What it does

Lets the spring develop concentrically, improving positional performance and isochronism.

How it works

Raising the terminal coil clear of the spring body allows it to breathe about its centre rather than pulling to one side.

Forming and adjusting an overcoil is highly skilled hand work.

Evolution

Retained in high-grade watchmaking alongside flat springs with computed terminal curves.

Known variations

  • Breguet overcoil
  • Flat spring with Phillips terminal curve

Identification for collectors

  • Visible as a raised outer coil above the spring's plane.

Common failure modes

  • Distortion from mishandling
  • Contact with the balance cock

Diagram

balance wheel with timing screwsflat or overcoil hairspringregulator index
Schematic: balance, hairspring and regulator. An outline schematic drawn to show the arrangement of the parts discussed here. It is not a dimensioned working drawing and does not depict a particular maker's calibre.

Sources & references

  1. de Carle, D., Practical Watch Repairing
  2. de Carle, D., Watch and Clock Encyclopedia
  3. Daniels, G., Watchmaking

Where sources disagree, the disagreement is stated rather than resolved silently.