The cal.3186 has exactly the same features as seen on the previous cal.3185, so there is no quickset date feature added as some guessed. The main difference between these calibers is the more precise setting of the 12 positions for the 12-hourhand when switching to another timezone.
The 12 hourhand clicks into the 12 positions without any play, and very precise and smooth, as the fixed positions where the hand clicks into, were moved from the center of the movement into the driving train of the hands, this eliminates the play that the older caliber has.
This resulted into a slightly higher caliber, from 6.4mm to 6.8mm, which resulted into a slightly higher case for the new GMT Master II when compared to the older GMT Master II.
Also, in this new GMT Master is the new, own hairspring fitted, the "
arachrom Bleu". This new Rolex-patented hairspring is very easy to recognise as the spring has a very dark blue shiny color. It consists of an alloy made of Niob (most part), Hafnium and a very small ammount other metals not described. This blue color is created because the Niob oxidizes in contact with air and formes a protective layer this way.
In comparison to the regular hairsprings, this new one consists of only non-Ferro materials, and is as such completely unsensitive to magnetic fields. Another major advantage is that this new spring is much less sensitive to temperature-variations, the frequency does not vary due to differences in temperature, which results into one of the most accurate mechanical watches you can buy today...