Differences Between Omega 8500a, 8500b, 8500g, 8800, 8900 Co-Axial Movements

When researching the purchase of my Omega Aqua Terra Skyfall (photographed below), one thing that greatly confused me was the caliber numbers and branding surrounding Omega’s 8000 series co-axial movements. Looking back, it seemed that I was not alone; even my local Omega AD was not able to explain the differences between confusing terms like “co-axial master chronometer” and “master co-axial chronometer” and their correspondence to caliber numbers. After researching extensively online, I think I’ve finally got it all straight (if not, correct me in the comments). Let’s go “beyond the dial” and take a look at the distinctions.

Variations of the Caliber 8500

To further complicate things, there are actually three versions of the caliber 8500: the 8500a, 8500b, and 8500g (sometimes also called the 8500c). The original 8500a was released in 2007 as an upgrade to the caliber 2500. Among other things, it featured a longer power reserve, improved shock absorption, better finishing, and a function to quickly switch between time zones without hacking the watch.

Omega then released the 8500b around 2011, with the primary advantage over the 8500a being the use of a silicon non-magnetic balance spring. The Aqua Terra Skyfall you see above, released around the same time as the premiere of the James Bond Skyfall movie in 2012, uses the 8500b.

Finally that takes us to the 8500g. Released in 2013, this caliber further extends the non-magnetic properties of the 8500b, with additional components being made from anti-magnetic materials. It has magnetic resistance rated at 15,000 gauss (hence the name 8500g).

So now the question you probably care about most: how do you identify whether your watch uses the caliber 8500a, 8500b, or 8500g? There are two places to check: the dial and the pictogram card that came with your watch. If the dial has the words”MASTER CO-AXIAL” on it instead of “CO-AXIAL”, your watch uses the caliber 8500g (this is what the phrase “master co-axial chronometer” refers to). If the dial says “CO-AXIAL” and you see a pictogram of a balance spring with the words “Si 14” on it, your watch uses the caliber 8500b. If the dial says “CO-AXIAL” and you don’t see that pictogram, your watch uses the caliber 8500a.

METAS and the Caliber 8800/8900

Ok, that wraps up the confusing part. Before we discuss the caliber 8800 and 8900, we first have to talk about METAS. METAS is the Swiss Federal Institute of Metrology, an independent company that Omega has partnered with to test watches to more stringent specifications than those for the chronometer certification given by COSC. A watch that passes these tests is given the certification as a “master chronometer”. According to Omega, these tests are:

  1. Function of COSC-approved movement during exposure to 15,000 guass magnetic field
  2. Function of watch during exposure to 15,000 gauss magnetic field
  3. Deviation of daily chronometric precision after exposure to 15,000 gauss
  4. Average daily chronometric precision of the watch
  5. Power reserve
  6. Deviation of chronometric precision in six positions
  7. Deviation of chronometric precision between 100% and 33% of power reserve
  8. Water resistance

Thus, all co-axial master chronometer watches are also master co-axial chronometers. The caliber 8800 and 8900 were the first two to receive “master chronometer” METAS certification. The caliber 8900 is functionally the METAS certified version of the caliber 8500g. The caliber 8800 is a new movement (not derived from the 8500) that is slightly different than the 8900, most noticeably with the date moved to the 6 o’clock position. Both of these movements can be easily identified by looking at the automatic winding rotor.


To summarize, here is a table comparing these movements:

CaliberSilicon Balance SpringAnti-magnetic to 15,000 Gauss (Master Co-Axial)METAS Certified (Master Chronometer)