overcoil

Overcoil is a term used in horology to describe a specific geometry of the hairspring in mechanical watches. In an overcoil design, the outermost turns of the hairspring are bent upward or extended beyond the main coil. This configuration aims to improve isochronism and reduce rate errors caused by positional effects.

The overcoil originated in the work of Abraham-Louis Breguet in the late 18th to early 19th century and became widely associated with his innovations. The form is often referred to as the Breguet overcoil, and it was adopted to help the hairspring return more consistently to its centerline across different positions, thereby enhancing accuracy.

Design and function: The raised outer coil helps to keep the hairspring concentric as it expands and contracts, guiding its ends and mitigating lateral deflection. Achieving the desired performance requires precise shaping, tempering, and alignment during manufacture. While the overcoil can improve rate stability, it also adds complexity and can make the hairspring more delicate and sensitive to shocks or magnetism, depending on the materials used.

Modern status: In contemporary watchmaking, overcoil hairsprings are still used in many high-quality timepieces, though advances in materials and designs—such as silicon or flat/harmonic springs—offer alternative approaches to achieving similar isochronism and resistance to positional errors. Some brands customize the overcoil geometry to balance durability with precision.

See also: hairspring, isochronism, Breguet overcoil.