Seikonaut 40mm Case – Black
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- Genuine 316L Steel & Sapphire
The Black Seikonaut takes the ultra-thin Nautilus silhouette into stealth territory with a 20-micron black PVD coating over 316L stainless steel. At 9.95 mm with a solid caseback, this 40 mm case delivers the same slim wrist presence as the brushed silver version but with a darker, more understated character. The sapphire crystal is available with clear or blue AR coating, and the screw-down crown provides 50 m water resistance. Compatible with NH35, NH36, NH38, NH70, NH71, and NH72 movements using 30.8 mm dials — a stealth Nautilus build starts here.
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 with 20-micron black PVD |
| Finish | Brushed and polished black |
| 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. Black, dark gray, and dark blue dials create a monochromatic stealth look. White or bright-colored dials create a high-contrast tuxedo effect against the black case.
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 steel, 20-micron black PVD)
- 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
Black-coated watches occupy a specific niche in the modding community — they read as tactical and understated, which is the opposite of the typical Nautilus aesthetic. That contrast is exactly the point. The Seikonaut’s brushed-and-polished finishing becomes even more nuanced in black PVD: the brushed surfaces absorb light while the polished edges catch it, creating subtle definition that flat matte cases lack. The 20-micron PVD is applied via Physical Vapor Deposition for molecular-level adhesion, making it harder and more scratch-resistant than painted or dipped coatings. At 9.95 mm thick, this is a stealth build that keeps the slim proportions intact.
Build Guide
Difficulty: Moderate — suitable for first-time builders with proper tools and patience.
Tools needed: Case opener (or case press), hand press or hand setting tool, stem cutter, movement holder, rodico, and tweezers.
Black PVD tips:
- Scratches show more: Black PVD reveals scratches more visibly than bare steel or lighter coatings. Use plastic or brass tools during assembly and keep metal tools away from the case exterior.
- Stem cutting: Measure twice, cut once. Leave 0.5 mm extra and trim down gradually.
- Gasket seating: Both the crystal gasket and caseback gasket must sit flat in their channels. A twisted gasket compromises the 50 m water resistance rating.
For full step-by-step instructions, see our complete Seiko mod build guide.
Frequently Asked Questions
Does the black PVD scratch easily?
The 20-micron PVD coating is harder and more scratch-resistant than standard plating or paint. However, scratches on dark coatings are more visible than on bare stainless steel because they reveal the lighter metal underneath. With normal daily wear, the finish holds up well. Use care during assembly and avoid abrasive surfaces.
What dial colors work best with a black case?
For a full stealth build, use a black or dark gray dial with black or gunmetal hands. For high contrast, a white or silver dial creates a striking tuxedo look. Green and blue dials also pair well, adding a touch of color against the dark case without breaking the overall subdued tone.
What movements fit the Black Seikonaut?
All standard Seiko NH-family automatics: NH35, NH36, NH38, NH70, NH71, and NH72. The NH35 is the most popular choice for daily-wear builds, while the NH72 skeleton pairs with the exhibition caseback for a see-through build.
Is this case waterproof?
The Black Seikonaut is rated to 50 m water resistance when assembled correctly with all gaskets properly seated. This covers hand washing, rain, and light splashes. For submersion, have a watchmaker pressure-test the completed build.
Complete Your Build
- Silver Nautilus Skeleton Dial (30.8 mm) — high contrast against the black case
- Silver V2 Nautilus Hands — bright hands for legibility against a dark build
- Seiko NH35 Movement — reliable daily-wear movement, the most popular choice for Seiko mods
- Seiko NH70 Skeleton Movement — open-heart movement for a partially skeletonized stealth build
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