The Rolex Caliber 1555/1556

It will seem strange to anyone with even a passing acquaintance with Rolex that the fiercely self-sufficient Swiss watchmaking giant didn’t start manufacturing its own calibers until 1957. Until then, the movements in its ever expanding and increasingly popular catalog had been sourced from the likes of long-term partner Aegler.

That all changed with the introduction of the 1500 series, the first mechanisms made completely in-house by Rolex themselves. Starting with the base model, the Cal. 1530, the range grew to consist of a slew of variations on its basic architecture to cover the diverse functionality of the brand’s watches.

With Rolex notoriously making very few models with radical complications, several of the 1500 series were able to find homes inside more than one famous name. The long-running Cal. 1560 and 1570 for example, powered everything from the Explorer to the Submariner to versions of the Datejust—simple, three-hand watches with nothing more demanding than a date function to contend with.

When Rolex did decide to come up with a more complicated piece however, it was with their typical obsessiveness for detail and a healthy dose of panache.

One and Only

The Rolex Day-Date, perhaps better known by its unofficial nickname The President for its long and illustrious association with the great and the good, was first seen just a year before the introduction of the new range of movements. To that point the only watch from any manufacturer to simultaneously present both the date and the day of the week spelled out in full, the President quickly took the crown as the brand’s flagship creation.

With its unique complication, the Day-Date was in need of a caliber all to itself. While the first two references in the series continued to use third-party movements, by 1959, Rolex had perfected the Cal. 1555; a variant of the 1530 with the added utility necessary to drive the second calendar display.

The ref. 1803 ushered in the next generation of the Day-Date. With its upgraded engine affording a slimmer and more graceful profile than its predecessors, it still had a substantial weight and presence thanks to the all precious metal construction—a tradition that continues to this day, nearly sixty years down the road.

Although the new caliber had the same diameter as the rest of the 1500 series at 28.5mm, it was the thickest out of the entire range, measuring 7.03mm top to bottom. The 18,000 VPH frequency gave the Day-Date a five ticks per second beat, not quite the smooth sweep of later models, but still highly accurate and steadfastly reliable.

Rated as a 25-jewel movement, it actually contained an additional three or four jewels in the calendar mechanism that, while not the handy Quickset version that made its debut in the seventies, benefitted from the instantaneous midnight date change system still used by Rolex today.

In common with its sibling calibers, the 1555 shared the same setup of free-sprung Nivarox hairspring with Breguet overcoil and a balance wheel regulated by Rolex’s patented Microstella screws. Protected by KIF Flector shock absorbers, it used a traditional stone lever escapement and had a power reserve of 42 hours.

The 1555 was produced from 1959 to 1967, overlapping in some models of the Day-Date with its successor, the Cal. 1556, released in 1965.

Ostensibly identical, the only major difference between the two movements was a considerable rise in balance frequency. The later caliber followed the example set by the rest of the 1500 series and saw its rate increased to 19,800 VPH. As well as a marginal improvement in accuracy, the higher beat gave the mechanism an added imperviousness to shocks.

In addition, the 1556 was rated as a 26 jewel movement, with the top end of the center wheel receiving the extra stone, although it too had supplementary jewels in the calendar that weren’t included in the official count.

At the start of the seventies, the whole of the 1500 family of calibers received their biggest update when the hacking function was introduced. Now, pulling out the winding crown stopped the seconds hand, an innovation intended to help make setting the exact time easier.

The 1556 was the last of the series to power the Day-Date. It was replaced in 1978 by the Cal. 3055, which brought with it the convenience of the Quickset date change to the newest version of the President, the ref. 18038.

In all, the 1555 and 1556 powered more than a dozen different models of Rolex’s crowning achievement. With the exquisite design housing the ultra reliable workhorse, they formed an impeccable partnership for nearly 20 years.