Hacking seconds

Hacking seconds is the ability to stop a running watch by pulling the crown out to its setting position. With the movement halted, the seconds hand can be aligned exactly with a reference such as a radio time signal or a phone clock, and the watch restarted at the moment the reference passes the minute. The alternative name, stop seconds, describes the same thing.

You meet it at the last crown click. On a typical three-hand automatic the crown winds when pushed home, corrects the date at an intermediate position, and sets the hands at the outermost one. If the watch hacks, the seconds hand stops the instant you reach that outermost click and starts again the instant you push the crown back in.

How it works

The mechanism is usually startlingly simple. A thin lever, moved by the keyless works when the crown reaches the setting position, swings across and presses lightly against the rim of the balance wheel. Friction stops the oscillation, and with the balance still the escapement locks and the entire train freezes. Push the crown home and the lever retracts, the balance is released, and the watch restarts, usually within a beat or two.

The forces involved are tiny. The lever tip meets the outer edge of the balance rim, where light contact is enough to absorb a swing of 270 degrees or so almost at once. Variations exist. Some movements brake the balance with a fine hairbrush-like tuft rather than a lever tip, some stop the fourth wheel or the seconds pinion instead of the balance, and Seiko's Spring Drive calibers, which have no balance at all, stop the glide wheel electromagnetically.

A smaller group of movements goes one step further with a zero-reset seconds, where pulling the crown snaps the seconds hand back to twelve rather than simply stopping it. That removes the wait for the reference to come round, but it is a much more involved mechanism and is rare outside expensive watches.

Where it comes from

The word comes from military practice. A time hack is the moment a unit synchronizes its watches to a single reference, and it is impossible to do properly with watches that cannot be stopped. United States military watch specifications of the 1960s, MIL-W-3818B among them, called for the feature by name, which is why the field watch lineage that produced watches like the Hamilton Khaki Field Mechanical is bound up with it.

Civilian adoption was gradual rather than sudden. Rolex, for instance, added stop seconds with the caliber 1570 in the mid-1960s; the 1560 it replaced ran on regardless of where the crown was.

Which movements hack

Most current mechanical calibers do. The ETA 2824-2 and its Sellita equivalent hack, as do the Seiko NH35 and 4R36, the Miyota 9015 and almost every Swiss automatic in current production.

The exceptions are worth knowing before you buy. The Miyota 8215 does not hack. Seiko's older 7S26, still found in large numbers on the used market, neither hacks nor hand-winds. Many vintage movements have no stop lever either. None of this is a defect, and none of it can be added later without changing the movement.

Why it matters

It matters only for setting precision. Without hacking you cannot synchronize to better than a few seconds, because the watch keeps running while you fiddle with the hands. With it, you can set a watch to the second and then judge its daily rate honestly over the following week, which is the only way to know how your watch actually performs.

The procedure is short. Wind the watch fully first so it is not running down while you measure, pull the crown out, set the minute hand a minute or two ahead of the reference, wait for the reference to reach the top of that minute, then push the crown in. Record the time and the date. A week later the difference, divided by seven, is your daily rate in seconds a day. Our pages on setting and adjusting your watch and testing accuracy at home take that further, and timing your own watch covers what a timing machine adds.

Common misconceptions

A watch that does not hack is not broken. Many excellent movements, including a good number of vintage calibers and some current ones, simply do not include the lever. Nothing is wrong and nothing needs repair.

Stopping the watch this way is also harmless. The balance is halted by gentle friction, not by force, and leaving a watch hacked for a period does the movement no damage, though it does let the mainspring sit at whatever tension it had and stops the date changing over.

Hacking does not make a watch more accurate. It changes nothing about the rate; it only lets you start from zero error rather than from an unknown one. Nor is it the same as a quickset date, which lives at a different crown position and does a different job, though the two are often listed together on a spec sheet. Finally, most quartz watches stop their seconds hand when the crown is pulled as well, because the circuit simply cuts the stepper motor. That is the same convenience arrived at by an entirely different route.