Component record · Regulation

Curb Pin

Verified

One of a pair of small pins, mounted on the regulator index, between which the outer coil of the balance spring (hairspring) passes.

What it does

Effectively shortens or lengthens the active length of the balance spring as the regulator index is moved, thereby speeding up or slowing down the balance's rate.

How it works

The regulator index, marked with a scale (often 'F' to 'S' for fast/slow, or a numbered arc), is pivoted so that moving it slides the curb pins along the hairspring's outer coil.

Moving the pins toward the stud effectively lengthens the free, vibrating portion of the spring, slowing the balance; moving them away shortens the active length and speeds it up.

The gap between the two pins must be close enough to grip the spring's action without excessive play, but not so tight as to pinch or distort the coil, which would harm timekeeping (a fault called 'shake' if too loose, or 'pinching' if too tight).

Evolution

Curb-pin regulation via a sliding index remains a standard, if imprecise, method of adjusting rate; many high-precision and modern movements instead favour free-sprung balances with adjustable balance-spring stud or weighted screws to avoid the friction curb pins introduce.

Known variations

Simple two-pin curb on a swan-neck or straight index
Boot-and-pin combinations that add a sliding boot alongside the pins for finer control

Identification for collectors

Two small pins visible straddling the hairspring near the balance cock, moved by the regulator index arm

Common failure modes

Excess play ('shake') between pins and spring, degrading rate stability
Pins pinching the spring too tightly, distorting its coil and causing poised or positional rate errors

Kieninger-Regulator (2022-05-02 Sp) — Lothar Spurzem, CC BY-SA 2.0 de. Wikimedia Commons file page

Sources & references

Daniels, G., Watchmaking

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