Seikonaut Watch Case - 38mm [ULTRA THIN EDITION] - Solid - - - Lucius Atelier - Swiss Quality Seiko Watch Mod Parts

Seikonaut 38mm — Ultra Thin Watch Case

Name: Seikonaut 38mm — Ultra Thin Watch Case
Brand: Lucius Atelier
SKU: SNC38SSUT/NH.Solid
Price: 120.0 SGD
Availability: InStock

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$120 SGD - $145 SGD

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A Compact Powerhouse. Reborn Ultra Thin. Big Presence.

Introducing the Seikonaut 38mm, part of our Ultra Thin Edition series — an iconic reinterpretation of a legendary sports-luxury silhouette, now engineered to be among the thinnest NH-compatible watch cases ever made.

The Seikonaut aesthetic — defined by its angular case geometry, sculpted flanks, and sporty elegance — has achieved cult status in the watch modding world. Coveted for its versatility and bold wrist presence, it blends refined design with everyday wearability.

While the bezel measures 38mm, the case tells a bigger story. Factoring in the flared side flanks, the full case width reaches 42mm, and with a 7mm crown installed, approximately 44mm. A commanding presence that still wears balanced — a bold statement piece that looks equally at home on small and large wrists.

Built around your choices. This is a case to build on. You choose the crown, the chapter ring, the dial, the hands and the movement — and because every Lucius Atelier case shares the same 28.5mm dial standard, the parts you'd buy for a Bauhaus 33 or an Explorer 36 fit the Seikonaut too. One parts ecosystem, one set of tolerances. The flat sapphire crystal is the one thing already done for you — it ships pre-installed and aligned at our workshop, so there's no crystal-pressing at your bench.

Wears 38, lives bigger.
Bezel diameter 38mm, full case width 42mm with the flared flanks, around 44mm with a 7mm crown. Lug-to-lug 47mm, lug width 20mm. A bold sports-luxury silhouette on a compact canvas.

Genuinely thin.
10.0mm with the solid caseback. The engineering behind that number: a standard NH movement is 7.59mm tall on its own — which leaves barely 2.4mm for the crystal, case walls, and hand clearance combined. Tight engineering.

Built in 316L steel.
Crafted from robust 316L stainless steel with a sleek brushed finish — enduring durability, tool-watch presence. Choose your preferred 7mm crown style: coin-edge, knurled, or onion.

Bracelet, your call.
The 20mm lug takes any standard 20mm strap or bracelet. We recommend our Seikonaut Bracelet 20/16mm — a sculpted 316L bracelet engineered around this exact lug profile — but every other Lucius Atelier bracelet fits too.

Caseback	Total Thickness	Water Resistance	Best for
Solid · HEADLINE BUILD	10.0mm	100m / 10 ATM	The thinnest profile — daily wear
Sapphire	10.9mm	100m / 10 ATM	Movement display — show off the rotor

The solid caseback is the headline build — this is the case at its slimmest. The sapphire caseback adds slight thickness in exchange for a view of the NH movement.

Heads up — sold separately

Crown. The crown shown in product photos is for visual reference only. Crowns must be selected above — choose your preferred 7mm style (coin-edge, knurled, or onion). Browse compatible crowns .

Chapter ring. This case requires a SKX013-spec chapter ring to seat the dial correctly — without it, the dial sits too low and hands won't clear. The case fits both normal and ultra thin chapter rings; the choice depends on your dial, not the case. If your dial has applied (raised) indices, choose ultra thin for clearance. If flat-printed, either works. When in doubt, ultra thin is the safer default. Browse compatible chapter rings .

For experienced modders only — TYPE M movements only

This case fits TYPE M versions of NH35, NH36, NH38, NH70, NH71, NH72 only. TYPE L will NOT fit. Due to tight clearance between hands and sapphire, this case is recommended only for professional watchmakers and experienced modders.

Specs
Build Guide
What's Included
How To Mod

Dimensions
1. Bezel Diameter 38mm
2. Case Width (with flanks) 42mm
3. Case Width (with crown) 44mm
4. Case Thickness 10.0mm (Solid Caseback) / 10.9mm (Sapphire Caseback)
5. Case Weight 49.4g (316L Steel)
6. Lug to Lug 47mm
7. Lug Width 20mm
Materials & Finish
1. Quality Swiss Production Quality, ISO 9001 Certified
2. Case Material 316L Stainless Steel
3. Case Finishing Brushed
4. Crown 7mm, screw-down (coin-edge, knurled, or onion — per selection)
5. Crystal Flat Sapphire with clear anti-reflective coating
6. Crystal Size 29.5mm x 1mm
Compatibility
1. Movements TYPE M: NH35, NH36, NH38, NH70, NH71, NH72
2. Dials 28.5mm
3. Chapter Rings SKX013-spec (required, sold separately)
4. Hands NH movement compatible
5. Crowns 7mm SKX-style
6. Bracelets 20mm lug — Seikonaut Bracelet 20/16mm recommended; fits all Lucius Atelier bracelets and any standard 20mm strap
Water Resistance
1. Solid & Sapphire Caseback 100m / 10 ATM
New to Seiko modding? Here's everything you'll need for a complete, functioning watch — in the order you'd actually assemble it.

What's already in this purchase:

The case body with pre-installed flat sapphire crystal, and your chosen caseback. That's the foundation. Everything below is what you'll add to make a wrist-ready watch.

The 5 parts you'll need for a complete watch:
1. Chapter ring — required first. The chapter ring seats the dial at the correct height; without it, the dial sits too low and the hands won't clear.
  (SKX013-spec — sold on a separate page. See the "sold separately" panel above for normal vs ultra thin guidance.)
  Browse compatible chapter rings
2. Crown — 7mm SKX-style options (coin-edge, knurled, onion). Pick a style that matches the watch's character.
  (Required to add to cart. Selected from the dropdown above on this page.)
  Browse compatible crowns
3. Dial (28.5mm) — the face of the watch. The biggest aesthetic choice you'll make. Match it to your chosen movement (date / day+date / no-date dial).
4. Hands — paired with your dial. Most modders match lume colour across both hands and dial markers (BGW9 white-blue, C3 green, etc.).
5. Movement — NH35 (date), NH36 (day+date), NH38 (no date), or source your own. The movement is the engine of the watch.
Optional finishing touches:
1. Bracelet or strap (20mm lug) — we recommend our Seikonaut Bracelet 20/16mm to match this case, plus custom shell cordovan / full-grain cowhide leather straps.
2. Decorated bridge + rotor (only visible with sapphire caseback) — replace the stock NH movement bridge and rotor with our Côtes de Genève decorated set, viewable through the sapphire caseback. Note: our bridges fit only our rotors (not OEM Seiko rotors), so they're chosen as a pair. Recommended for experienced modders only — or tick Assemble Watch below and we'll handle it.
Want it built for you?

Tick the Assemble Watch (+SGD $50) option above on this page and we'll handle the full build in our Singapore workshop — case fitting, movement seating, crown alignment, caseback press, function check, and decorated bridge + rotor installation if you've added those.

Heads up: assembled watches require 2–5 business days of workshop time before shipping.

Once your parts arrive, see the How To Mod tab for assembly tutorials.
1. 1x Watch Case Body (with pre-installed flat sapphire crystal)
2. 1x NH Caseback (Solid or Sapphire, per selection)
All photos of watches with modification parts are for illustration purposes only and are not included with the case purchase.

How heavy is it?

The steel case alone is 49.4 grams — the reassuring tool-watch heft modders expect from a 316L sports case. If you want the same Seikonaut dramatically lighter, the Grade 4 titanium version is just 29 grams, over 40% lighter.

What comes in the box — do I need to buy a crown?

Yes, a crown is required and selected on this page. The Steel Seikonaut ships as the case body with a pre-installed flat sapphire crystal and your chosen caseback (solid or sapphire). You select your 7mm crown style — coin-edge, knurled or onion — from the dropdown above, and it's added to the build. To finish the watch you'll also need a dial, hands, chapter ring and movement.

Which bracelets and straps fit?

The Seikonaut 38mm uses a standard 20mm lug width, so it takes any 20mm strap or bracelet. We designed the Seikonaut Bracelet 20/16mm specifically for this case, but every other Lucius Atelier bracelet fits too — the case is a foundation, and how you finish it is yours to decide.

Why does the case look bigger than 38mm?

The bezel measures 38mm, but the Seikonaut's flared side flanks push the full case width to 42mm — and to around 44mm with a 7mm crown installed. It's a deliberate part of the sports-luxury silhouette: a bold wrist presence on a case that still wears balanced.

What's the difference between Type M and Type L?

Type M and Type L are Seiko's two factory hand-fitting versions of the NH series. Type L sits higher than Type M — about 0.4mm taller from base to second-hand pinion. Because of the tight clearance between hands and sapphire in Ultra Thin Edition cases, only Type M will fit. Confirm your movement type before ordering.

Do I need to remove the sapphire crystal to install the chapter ring?

No — and that's the whole point of us pre-installing the sapphire for you. The chapter ring sits on top of the dial inside the case, not under the crystal, so the crystal stays where it is throughout the build. (If you ever do need to remove the sapphire — for example, to replace it — that's a workshop job we handle for you, not an at-home step.)

Should I choose the normal or the ultra thin chapter ring?

The choice depends on your dial, not the case — both sizes fit every Ultra Thin Edition case. If your dial has applied (raised) indices, the ultra thin chapter ring is the better choice because its slimmer profile gives the indices clearance instead of resting on top of them. If your dial is flat-printed (no applied markers), either size works. When in doubt, the ultra thin chapter ring is the safer default because it works with both dial types.

What is an SKX013 chapter ring and why does it matter?

A specific size and spec used by Seiko in the SKX013 (discontinued 2019). The LA smaller-case lineup all uses this same spec, which means dozens of chapter ring designs work across every smaller case in the catalogue.

Will SKX007 dials fit?

Any 28.5mm dial will fit. Stock SKX007 dials may seat differently because the original SKX007 has a 4 o'clock crown position with offset dial feet, while Ultra Thin Edition cases are designed for 3 o'clock dial feet.

Does this case need a chapter ring?

Yes — every Ultra Thin Edition case requires an SKX013-spec chapter ring, sold separately. The chapter ring seats the dial at the correct height; without it, the dial drops and the hands will not clear properly.