Hacking Seconds

Hacking (or hack seconds) stops the balance wheel at the exact moment the crown is pulled out to the time-setting position. It is not a complication in the classical sense: it is a mechanical service device designed to do one thing with absolute reliability.

The physical principle is simple. When you pull the crown, a lever or blade spring comes into contact with the balance wheel — or, in certain calibers, with the seconds wheel — and locks it. The train remains under tension: the mainspring does not discharge, no time is lost. As soon as you push the crown back, the balance wheel starts again precisely from where it was. The real advantage is that you can wait for the time signal, then restart the movement at the exact second, without having to calculate how many seconds have passed since the last hand passage over zero.

In my workshop I have disassembled movements from the 1950s without this function: setting the time meant mentally counting and pushing the crown at the right moment, with a margin of error of a few seconds almost guaranteed. Hacking has eliminated that variable. It is no wonder it became a basic requirement for military and aviation watches: the British specification for the so-called W10 explicitly required it, and calibers like the Lemania 1872 were accepted partly because they featured it. For pilots, synchronization with the radio signal was not a convenience, it was a procedure.

From a construction standpoint, the most common solution acts on the lever or directly on the balance wheel via a lever arm connected to the crown tube. Some manufacturers — Nomos with the DUW 4001, Patek Philippe with caliber 26-330 — adopt proprietary geometries that reduce stress on the escapement wheel during the hack. Seiko introduced the function on caliber 4R36 in 2011 as an upgrade over the 7S36, which lacked it: that choice changed the perception of the mid-range Japanese automatic segment. Today the absence of hacking on a mid-to-high-priced automatic is almost always a negative note in technical reviews, even though there remain quality calibers — certain historical Valjoux movements, some vintage Longines automatics — that lack it without being any less precise in timekeeping.

One clarification worth making: balance wheel hacking and chronograph seconds reset are two distinct mechanisms. On a chronograph, the hacking of the center chronograph seconds hand is governed by the start/stop/reset mechanism, independent of the crown position.