Introducing Ref. STGK016, and the Story Behind the Grand Seiko Ladies’ Standard

STGK016 is the latest addition to Grand Seiko’s line of women’s watches. A limited edition of just 60 pieces, STGK016 establishes the “Classic Series” as a new design foundation for Grand Seiko women’s models, one that is inspired by the overall case and crystal shape of the very first Grand Seiko timepiece from 1960. It is rendered in 18k rose gold, measures 29.5mm in diameter and 10.5mm in thickness, features a distinct knurled bezel and twelve diamonds on top of a deeply textured dial, and runs on the automatic in-house caliber 9S27.

Despite its diminutive size, caliber 9S27 is a high-performance movement with an impressive mean daily rate of -3 to +8 seconds a day and 50 hours of power reserve. To achieve these specifications in a smaller movement, Grand Seiko’s engineers had to go back to the drawing board, beginning with caliber 9S25. Below, we’ll explore what went into creating this new standard for women’s watches.

The woman behind the development of 9S25 was Sayaka Yamada. Yamada joined Seiko Instruments Inc. (SII) in 2006. In 2009, she transferred to the W Product Planning Department of the Watch Division, and it was from her position there that in 2015 Yamada and her team began work on 9S25.

One of the primary goals in the inception and design of 9S25 was to achieve Grand Seiko’s standard for high precision, but to do so in a significantly smaller caliber. It is often the case that the smaller and thinner a movement becomes, the harder it is to stabilize its accuracy. This is largely because the balance wheel also needs to shrink down to fit within the smaller architecture of the movement, and a smaller balance is often not as resilient against external forces the way a larger one is.

Because of the movement’s size, the gear train layout had to be rethought entirely. Part of that redesign included reworking the teeth on each gear to ensure maximum energy transfer through the train. This new train also needed to leave enough room for the barrel, and though it was reduced to about 72% of the barrel size of a larger mechanical caliber, Grand Seiko was still able to achieve a 50-hour power reserve, in part due to the use of the brand’s proprietary spring alloy, otherwise known as Spron.

Altogether, the total number of parts in caliber 9S25 is 236, which exceeds the number in an equivalent Grand Seiko men’s caliber. In addition to ensuring rigidity of key components to support long-term operation, previously integrated parts had to be separated due to their small size, resulting in a higher parts count.

In charge of the assembly of all 50 units of 9S25 was a watchmaker by the name of Yuki Kudo. Kudo has been a member of Studio Shizukuishi since 2007, where she has worked on the assembly and accuracy adjustment of Grand Seiko men’s calibers.

Naturally, working with a notably smaller movement presented certain challenges. Kudo had to remake her tools, including her screwdrivers and tweezers, to deal with the smaller size of the componentry. This was especially important when working with a miniaturized balance spring, which has to be manually adjusted by a watchmaker to achieve optimal performance. A special tool also had to be developed to check the spacing between parts, because the traditional tool for doing so was far too thick to fit between the parts here.

Naturally, one of Kudo’s biggest challenges was working the balance. With the men’s 9S calibers, there is over 20 years of solid know-how and precedent in place, but such wasn’t the case here, so it was a process of trial and error to get the adjustments right. The tilt proved to be especially difficult given the caliber’s size. Altogether, much of the assembly had to be done with the aid of a microscope.

The 9S27 caliber powering STGK016, which features an exhibition case back, boasts the same performance specifications as 9S25. Unlike 9S25, however, 9S27 is a continuous production movement, and it will serve as the benchmark going forward for women’s mechanical timepieces.