Omega Caliber 3861

Manufacturer	Omega
Caliber Number	3861
In-House?	Technically no
Based On	Lemania caliber 1873
Movement Type	Mechanical, manual-wind
Chronometer?	Yes (Omega Master Chronometer verified by METAS)
COSC?	Yes
Diameter	27mm
Jewels	26
Beats Per Hour	21,600 vph, 3Hz
Lift Angle	Unconfirmed (use Co-Axial compatible timing machine)
Power Reserve	50 hours
Shock Protection	Nivashock
Escapement	Co-Axial
Balance	Free sprung
Balance Spring	Si14 Silicon
Anti-Magnetic?	Yes up to 15,000 gauss
Chronograph System	Cam style
Functions	Central hours; central minutes; central chronograph seconds; running seconds subdial at 9:00; 30 minutes chronograph counter at 3:00; 12 hour chrono counter at 6:00
Hacking Seconds?	Yes
Finishing	Rhodium plated finish or Moonshine gold plated; bridges with straight Geneva waves
Country of Manufacture	Switzerland, Swiss made
Known Models	Omega Speedmaster Moonwatch Professional Co‑Axial Master Chronometer Chronograph 42mm (Add your watch in the comments below…)

The Omega caliber 3861 is a handwound mechanical chronograph movement first introduced in the Omega Speedmaster Apollo 11 50th Anniversary watch. This caliber essentially replaces Omega’s use of the calibre 1861 and 861 be that. It is Swiss made with 27 jewels.

In Omega’s own words:

“Manual-winding chronograph movement with Co-Axial escapement. Certified Master Chronometer, approved by METAS, resistant to magnetic fields reaching 15,000 gauss. Free-sprung balance with silicon balance spring. Rhodium-plated finish, bridges with straight Geneva waves.” –source

Is the Omega 3861 an in-house movement?

The 3861 is based on the Lemania 1873, with many “upgrades” including an anti-magnetic hairspring and co-axial escapement. While Lemania was acquired by Swatch Group in 1999 via the purchase of Breguet, the in-houseness of the 3861 is up for debate due to the fact that it is overall technically based on the Lemania 1873. While some purists may not be willing to consider the 3861 as an in-house movement (designed and made by the manufacturer from the bottom up), it is quite close to being in-house as it has been modified from its original form by Omega, manufactured by Omega, and only used by Omega – which is controlled by the same watch conglomerate that also controls the company that designed the original. Short answer: the 3861 was not developed from scratch by Omega engineers. Share your thoughts on this below.

Rhodium VS Moonshine Gold:

There are two versions of the cal. 3861: one available in a silvery rhodium plated finish and the other in what Omega calls Moonshine Gold plated finish.

Disappointingly, Omega chose to allow for confusion by not giving each finish its own calibre number. Luckily, the Moonshine 3861 is not as widely available as the rhodium plated version, but if you have $36,500 to spend on a Speedy then it’s all yours (and George Clooney’s) – to be more specific, the 3861 Moonshine gold is found in the Speedmaster Anniversary Series Co‑Axial Master Chronometer Chronograph 42 mm (ref: 310.60.42.50.99.001).

All you other peasants must settle for a silver tone movement – even if you buy a Moonshine gold watch.

3861 VS 1861:

The 3861 is considered to be an update movement to previous Moonwatch models which were powered by the Omega caliber 1861 or 1863. The 3861 is an evolution to those as the 1861 has been discontinued. One of the main differences between the 1861 and 3861 would be the jewel count: the 1861 has 18 jewels and the 3861 has 27. The 1861 is non-hacking, whereas the 3861 has a stop seconds feature when setting the time. 3861 powered Moonwatches are the first of their kind to hack! Both calibers are about 27mm in diameter and beat at the same 21,600 bph. Although on paper, and in appearance, the two movements are similar overall, more than half of one caliber is not interchangeable with the other.

The gif above shows how each movement looks from the caseback view. Notice the Omega text on the 1861 is upside-down when flipping the watch over (that’s why making this video was a little tricky on the first attempt).

Editor’s observation: Clever how in the stock images of the two movements, one appears to have the chronograph engaged and one does not – giving the appearance of being more different in design than they really are. Look at the chrono reset hammer. Perhaps the brand thought themselves as being clever to produce such slight “differences”. Compare the 3861 image from the gif to the tilted movement image above.

Anti-Magnetic Properties:

It used to be that the Omega caliber 8508 got all of the credit for being resistant to magnetic fields, but now most of the newer movements such as the calibre 3861 being discussed here, are anti-magnetic up to 15,000 gauss thanks to the silicone balance spring and materials that are not affected by exposure to magnetism.

Replacement Prices:

The Omega caliber 3861 is an in-house movement and not available for individual purchase, even for Swatch Group parts accounts. At the time of this post, the retail prices of the watches this movement powers ranges from $8,300 USD to $14,000 USD.

Additional Resources:

• Omega Moonwatch 42mm spec sheet (pdf)
• More Omega watch calibers

Images credit: Omega SA – if you have better pics of the movement in your watch, please submit them here.