Short answer
No one has cloned Omega's Calibre 8605. What sits inside a first copy Omega GMT sold as an "8605" is a different automatic movement, usually an Asian GMT caliber, dressed up with an engraved rotor and striped bridges. It runs, it winds, and it can fool you through a display back at arm's length. The coaxial escapement, the twin mainspring barrels, and the 60-hour reserve are not there. So when you see "8605" in a listing, treat it as a cosmetic label, not a technical claim.
What the real Calibre 8605 is
The 8605 belongs to Omega's 8500 family, the movement line that carried the brand from 2007 onward. It is a full manufacture caliber with a GMT function, and its spec sheet is the reason a true clone is so unlikely.
- Co-Axial escapement, a three-level escape wheel with three pallet stones instead of the usual two
- Two mainspring barrels working in series, roughly 60 hours of reserve
- 25,200 vibrations per hour, or 3.5 Hz, so seven beats per second
- Silicon Si14 balance spring on later production
- Jumping local-hour hand for GMT setting, so you advance the hour without stopping the watch
- 39 jewels, rhodium plating, Cotes de Geneve striping, and an engraved rotor
What is actually inside a fake "8605"
Replica factories build to a price, and a coaxial escapement is expensive to make. Three base movements cover almost everything sold as an 8605:
- A 7750-derived Asian chronograph-base GMT, often from Shanghai or Hangzhou. It beats at 28,800 vph, runs one barrel, and usually tops out around 40 hours.
- A clone of the ETA 2893-2, the classic GMT workhorse. Reliable, 28,800 vph, no jumping hour, and a 24-hour hand that sets separately from the hour hand.
- A 2824 or 2836 clone with a bolt-on GMT module, which is cheap and adds thickness to the case.
- Quartz in the lowest tier, where the seconds hand ticks instead of sweeping.
The second hand is your fastest tell. Seven beats per second looks slightly slower and smoother than the eight beats of a 7750-based build.
How to check one before you pay
- Count the beats. Film the seconds hand for ten seconds and step through it. Eight ticks per second means a 28,800 vph stand-in, not an 8605.
- Run a power reserve test. Wind it fully, set it down, and check it at hour 45. A real 8605 family movement is still going. A 7750-based fake is usually dragging or stopped.
- Work the crown. On the genuine 8600-family GMT, the local hour hand jumps in one-hour steps. On most fakes the hour hand stays put and the 24-hour hand moves instead.
- Look at the escapement through the caseback. The coaxial wheel has a distinct stacked, three-tier look. Clone movements show a conventional flat escape wheel under a decorative plate.
- Check case height. GMT modules stacked on a base movement add millimeters. If the watch sits noticeably taller than the published spec, that is a module, not a manufacture caliber.
- Listen to the rotor. A decorated 7750 variant spins loudly and wobbles. The 8500 family winds quietly with two barrels absorbing the load.
Where the tiers land
Most first copy Omegas sort into four buckets. Quartz sits at the bottom. Above it are 2824 clones with GMT modules, then 2893-2 clones, then decorated 7750-derived builds with better dial printing, sapphire with anti-reflective coating, and cloned hands. I look at the case profile and the dial text before the movement, because that is what you see on the wrist. The movement only matters when you flip the watch over or hand it to someone who knows what a seven-beat sweep looks like.
An honest framing
If what you want is the coaxial escapement and the 60-hour reserve, no first copy delivers it, at any price. Buy the real watch. If you want the look of an Aqua Terra GMT or a Planet Ocean GMT, judge the fake on the things a replica can actually get right: case finishing, bezel action, dial texture, date font, hand shape, and bracelet feel. Accept that the movement is a stand-in, ask the seller which base caliber it uses, and decide whether you can live with 28,800 vph and a shorter reserve. That is the whole trade.