3185 — the GMT automatic that defined the modern GMT-Master II

In one line

The engine of the GMT-Master II 16710 and the Explorer II 16570 for the best part of twenty years. An independent hour hand, 31 jewels, and a flicker in the 24-hour hand that now identifies it without opening the case.

First things first: is yours a 3185 or a 3186?

Rolex swapped the calibre mid-run and never announced it. The GMT-Master II 16710 was built from 1989 to 2007 and the Explorer II 16570 from 1989 to 2011; the vast majority have the 3185, and the last of them left with the 3186. No official serial number marks the border.

You can check from the outside, without a screwdriver. Move the hour hand as though setting the watch, and keep your eye on the 24-hour hand.

That twitch is not a fault, nor a symptom of one. It is how the movement is built: the 3185 drives the date disc with a pawl mounted on the hour wheel, and that pawl has play in it. For the 3186 Rolex replaced it with a detent spring and the play went away.

A late 16710 or 16570 with a 3186 now carries a premium. A genuinely small one: on these references the condition of the case and bezel counts for far more than the calibre number. And be careful, because the test tells you what is inside now, not what left the factory. These watches have been passing through workshops for thirty years.

3185 versus 3186: what changed and what didn't

Rolex left the architecture alone. It changed two parts — the hairspring and the date drive — and in doing so removed the very quirk by which the two are told apart today.

The Parachrom hairspring is the better part and nobody argues otherwise: it shrugs off magnetism and, according to Rolex, takes shocks up to ten times better.

The detent spring has more critics. Some workshops hold that the 3185's pawl, play and all, gave less trouble in the long run than the part that replaced it. That is a bench opinion rather than a factory figure, but it is repeated often enough to be worth weighing.

Where it came from

In 1983 Rolex solved an old problem: changing country without losing the time at home. The calibre 3085, launched in the GMT-Master II 16760, was the first Rolex movement whose hour hand could be set on its own, an hour at a time, without disturbing the minute or seconds hands. It worked. It was also thick, and the case had to grow with it: a wider bezel, a bigger crown and a nickname that stuck, the “Fat Lady”.

The 3185 came in 1989 to put that right. Same function, 31 jewels instead of 27, 6.45 mm tall, and an Oyster back to normal proportions. It went into the steel GMT-Master II 16710, the steel-and-gold 16713 and the gold 16718. And, as plenty of people forget, into the Explorer II 16570.

It lasted seventeen years. In 2006 Rolex began slipping in the 3186 with its Parachrom hairspring, a trickle at first and then wholesale. The 16710 bowed out in 2007 and the 3185 went with it.

Three things almost nobody knows

• The same calibre did two different jobs In the GMT-Master II the 24-hour hand points at a rotating bezel and gives you a second time zone, or a third if you turn the bezel. In the Explorer II 16570 it points at a fixed bezel and does something else entirely: it tells you whether it is nine in the morning or nine at night. That sounds daft until you have spent three weeks in a cave, or somewhere the sun never sets.
• The “II” in the name is in the calibre, not the case A GMT-Master and a GMT-Master II look very much alike from the outside. The whole difference is mechanical. On the earlier watches, with the calibres 1575 and 3075, the hour hand and the 24-hour hand move together and the second zone is read by turning the bezel. From the 3085 onwards the hour hand jumps on its own. That Roman numeral is a movement, not a design.
• The twitch gives the date away What makes the 24-hour hand dance is the same part that pushes the date disc along: a pawl mounted on the hour wheel. Rolex changed it in the 3186 to take the play out and, without meaning to, created the test by which the two calibres are told apart across a table.

Technical data

What a watchmaker checks before buying one

The hand test tells you which calibre is inside. It does not tell you whether it is healthy. What you check is the hour hand's jump: it should move an hour at a time, in both directions, landing cleanly on the marker without dragging the minute hand along. If the minute hand shifts on each jump, there is wear in the train.

Then the date. It should change at midnight and change in one snap. A lazy change, or one that happens at noon, means something is not where it should be: sometimes it is only the hour hand set twelve hours out, sometimes it is the pawl.

And then the money. Rolex has spent years closing parts accounts to independent watchmakers who lack its certification, so a proper service on a 3185 almost always ends up at the official workshop. That fixes both the price and the wait, and it also means the workshop may replace parts with current specification. If originality matters to you, ask for the paperwork from earlier services before you pay.

Watches that used it

• GMT-Master II ref. 16710 — 1989-2007, steel; Pepsi, Coke and black bezels
• GMT-Master II ref. 16713 — steel and yellow gold
• GMT-Master II ref. 16718 — yellow gold
• Explorer II ref. 16570 — 1989-2011; the last of them already with the calibre 3186

From the same maker, and with the same insistence on doing everything in-house: the calibre 4130, the first chronograph Rolex designed from scratch.