Seikonaut 40mm Case – Silver
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- Genuine 316L Steel & Sapphire
At just 9.95 mm with a solid caseback, the Silver Seikonaut is one of the thinnest Nautilus-style Seiko mod cases available. CNC-machined from 316L stainless steel with brushed and polished finishing, this 40 mm case features an integrated-lug silhouette inspired by the iconic Nautilus shape. The included sapphire crystal comes with your choice of clear or blue AR coating, and the screw-down crown delivers 50 m water resistance. Compatible with NH35, NH36, NH38, NH70, NH71, and NH72 movements using 30.8 mm dials — read on for full specs, compatibility details, and build tips.
Specifications
| Specification | Value |
|---|---|
| Diameter | 40 mm (10H–4H) × 38 mm (12H–6H) |
| Thickness | 9.95 mm (solid caseback) / 10.3 mm (exhibition caseback) |
| Material | 316L Stainless Steel |
| Finish | Brushed with polished accents |
| Crystal | Sapphire (clear or blue AR coating) |
| Water Resistance | 50 m |
| Dial Fit | 30.8 mm |
| Crown | Screw-down, included |
Compatibility
Movements: NH35, NH36, NH38, NH70, NH71, NH72 — all standard Seiko NH-family automatics fit without modification. The crown stem must be cut to length during installation.
Dials: 30.8 mm dials only. This case does not accept 28.5 mm dials without a spacer ring.
Strap/Bracelet: Compatible with the Seikonaut integrated bracelet. Standard spring bar attachment at the lugs.
Not compatible: 7S26 and 7S36 movements use a different stem height and are not recommended for this case.
What’s Included
- Seikonaut case body (316L stainless steel, brushed/polished)
- Caseback (solid or exhibition, depending on variant selected)
- Screw-down crown with signed logo
- Crystal gasket and caseback gasket
- Sapphire crystal (pre-installed, clear or blue AR coating)
Tools are not included. You will need a case opener, hand press, stem cutter, movement holder, and rodico to complete your build.
Why We Chose This Case
Most Nautilus-style mod cases on the market sit between 12 mm and 14 mm thick, which gives them a bulky wrist presence that strays from the slim profile of the original design. The Seikonaut achieves 9.95 mm by re-engineering the internal tolerances — tighter gasket channels, a thinner crystal seat, and a case mid-section milled to precise clearance above the movement. Every case is dimensionally checked before shipping. The integrated-lug silhouette flows into the bracelet line without the stepped transition found on budget alternatives, and the brush-to-polish finishing matches the language of high-end integrated-bracelet sports watches.
Build Guide
Difficulty: Moderate — suitable for first-time builders with the right tools and patience.
Tools needed: Case opener (or case press), hand press or hand setting tool, stem cutter, movement holder, rodico, and tweezers.
Common mistakes to avoid:
- Stem cutting: Cut the crown stem too short and the crown will not engage the movement. Measure twice before cutting, and leave 0.5 mm extra to trim down.
- Gasket placement: Both the crystal gasket and caseback gasket must be seated flat in their channels. A twisted gasket will compromise water resistance.
- Dial alignment: Align the dial feet before pressing down. Forcing a misaligned dial can bend the feet or scratch the movement.
For full step-by-step instructions, see our complete Seiko mod build guide.
Frequently Asked Questions
What movements fit the Seikonaut case?
The Seikonaut accepts all standard Seiko NH-family automatic movements: NH35, NH36, NH38, NH70, NH71, and NH72. The crown stem needs to be cut to the correct length during installation. The older 7S26 and 7S36 movements are not recommended due to a different stem height.
How thin is the Seikonaut compared to other Nautilus-style cases?
At 9.95 mm with a solid caseback (10.3 mm with exhibition), the Seikonaut is significantly thinner than most Nautilus mod cases on the market, which typically measure 12–14 mm. This slimmer profile sits closer to the wrist and better captures the proportions of the original Nautilus design.
What is the difference between the exhibition and solid caseback?
The solid caseback is 9.95 mm total thickness and gives a cleaner, more traditional look. The exhibition caseback adds a sapphire window that lets you see the movement, bringing the total thickness to 10.3 mm. Both options maintain 50 m water resistance. If you are using a skeleton movement like the NH72, the exhibition caseback lets you view the mechanics from the back as well.
Is this case waterproof?
The Seikonaut is rated to 50 m water resistance when assembled correctly with all gaskets seated properly. This rating covers hand washing, rain, and light splashes. We recommend having a watchmaker pressure-test the completed build if you plan to submerge it.
Complete Your Build
- Silver Nautilus Skeleton Dial (30.8 mm) — matching silver tone, designed for the Seikonaut case
- Silver V2 Nautilus Hands — brushed silver hands to match the case finish
- Seiko NH35 Movement — the most popular automatic movement for Seiko mods, reliable and easy to source
- Seiko NH72 Skeleton Movement — pair with the exhibition caseback for a full skeleton build
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Nautilus-Style Seiko Mod Parts · All Seiko Mod Cases · Nautilus Seiko Mods
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We accept returns/exchanges according to our policy for unworn items in original condition. Custom changes may have different terms — contact us for questions
