Grand Seiko’s Spring Drive U.F.A. Ushio 300 Diver is the Most Accurate Automatic Watch Ever Made

By Ben McKimm - News

Updated: 6 May, 2026

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Readtime: 5 min

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• Grand Seiko launches its smallest Spring Drive diver’s watch at 40.8mm
• New Calibre 9RB1 features “Ultra Fine Accuracy” of ±20 seconds per year
• High-Intensity Titanium construction makes the case 30% lighter than stainless steel
• Dials feature a gradient “Ushio” tide pattern in blue or green
• A new locking extension clasp offers up to 24mm of total adjustment

We are taught not to judge a book by its cover, but when it comes to watches, only a few seconds on the wrist can leave a lasting impression. That was my experience with the Grand Seiko diver at Watches & Wonders 2026. But the more time I spent thinking about it and looking through the images I took and the videos I shot at the event, the more I came to understand the significance of this release.

Traditionally, the Grand Seiko diver has favoured a substantial wrist presence, often pushing case diameters toward 44 mm. This design philosophy prioritised underwater legibility and professional-grade durability, yet it often sidelined collectors with smaller wrists. Now that the industry has firmly shifted back toward compact, wearable dimensions, the Japanese manufacturer has responded by refining its flagship sports line for a broader audience. Despite the high-performance internals, the brand managed to shave the case diameter down to 40.8mm and the thickness to a relatively svelte 12.9mm.

The headline figures for the new Spring Drive U.F.A. Ushio 300 Diver’s (ref. SLGB023 and SLGB025) are the Calibre 9RB1 movement. It’s a Spring Drive unit that boasts “Ultra Fine Accuracy” (U.F.A.) of ±20 seconds per year. This level of precision dwarfs the mechanical standards of a Rolex Submariner or an Omega Seamaster, and it’s achieved through vacuum-sealed crystal oscillators and sophisticated thermo-compensation. Most impressively, the water resistance remains professional-grade at 300 metres.

Ushio Dial Texture and Evolution 9 Geometry

The “Ushio” (tide) dial theme returns, drawing inspiration from the waters surrounding the Japanese archipelago. Unlike previous flat executions, these references utilise a gradient design to mimic the way light penetrates varying ocean depths.

Reference SLGB023 captures the deep blue of the open sea while reference SLGB025 reflects the lighter greens of shallow coastal waters.

Consistent with the Evolution 9 design language, the case features a lower centre of gravity and broader lugs to ensure the watch sits flush against the wrist. The High-Intensity Titanium surfaces are finished with Zaratsu polishing, creating a distortion-free mirror finish that contrasts against the hairline-brushed segments. Diamond-cut, multi-faceted indexes and powerful Lumibrite-filled hands ensure legibility remains, even in low-light environments. Finally, the ceramic bezel insert provides 120 clicks of precise adjustment and is naturally resistant to the corrosive effects of salt water.

Calibre 9RB1 and Ultra Fine Accuracy

What appears to be a beautifully finished Grand Seiko with a clever movement transforms into something else the more time you spend with it. Still, the heart of these timepieces is what you’re mostly buying them for, and the new Calibre 9RB1, hand-assembled by the Shinshu Watch Studio, delivers.

This movement builds upon the U.F.A. designation introduced in 2025, refining the Spring Drive system to reach an annual accuracy rate that most brands only achieve with pure quartz.

The technical prowess of the Calibre 9RB1 is anchored by several key engineering advancements. To ensure its “Ultra Fine Accuracy,” the movement utilises a vacuum-sealed crystal oscillator that protects the internal mechanisms from the adverse effects of temperature, humidity, and static electricity. This is further supported by a low-power IC that provides precise thermo-compensation by utilising frequency data gathered across multiple temperatures.

You’ll find the 72-hour power reserve indicator repositioned on the dial side for improved tracking and legibility. Additionally, a dedicated regulation switch allows technicians to adjust the movement during long-term service, ensuring the timepiece maintains its exacting standards over decades of ownership.

Price and Availability

Both the SLGB023 and SLGB025 are priced at AUD$17,700.

These models are not listed as limited editions, and they will be available through Grand Seiko Boutiques and select retail partners starting in June 2026. Each watch includes the manufacturer’s standard warranty and a high-intensity titanium bracelet.

Frequently Asked Questions

How does the “U.F.A.” designation affect the watch’s accuracy?

The Ultra Fine Accuracy designation guarantees an annual rate of ±20 seconds, making it the most accurate mainspring-powered movement in the world as of 2026.

Is the titanium used in these watches different from standard titanium?

Grand Seiko utilises High-Intensity Titanium, which is 30% lighter than steel and significantly more scratch- and corrosion-resistant.

Does the new diver feature a micro-adjustment clasp?

Yes, the new locking extension clasp allows for 6mm of fine-tuning for daily wear and a total extension of 24mm.