The Seikonaut 40mm is one of the thinnest Nautilus-style Seiko mod cases available — just 9.95 mm at the solid caseback. CNC-machined from 316L stainless steel with brushed and polished surface finishing, this 40 mm case features an integrated-lug silhouette inspired by the iconic Nautilus shape, available in six finishes (Silver, Black, Gold, Rose Gold, Two-Tone Gold, Gunmetal). 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 finish guidance. Available in 6 finishes Jump to: Silver · Black · Gold · Rose Gold · Two-Tone Gold · Gunmetal Silver — brushed stainless with polished accents The cleanest, most versatile finish. Brushed 316L surfaces with polished bevel transitions catch light without showing wear. The all-purpose choice that pairs with any dial and any strap. Black — full PVD-coated stealth finish A full physical-vapor-deposition coating across the case body, bracelet, and caseback. The matte tonality reads dressy-stealth in daylight and disappears entirely in low light. Pairs strongly with skeleton dials and dark-toned hands. Gold — warm yellow champagne tone Yellow-gold PVD with a champagne warmth in the brushed surfaces. The dressiest of the warm-tone finishes; pairs traditionally with cream or champagne dials and brown leather, more boldly with deep blue or black-skeleton dials. Rose Gold — soft pink-gold dress tone Pink-gold PVD applied at lower colour-saturation than the yellow variant — reads warm and understated, the dressiest of the six finishes. Pairs especially well with white, ivory, or chocolate dials. Two-Tone Gold — brushed steel with polished gold accents A duo finish — brushed silver case body with polished gold bezel, crown, and centre bracelet links. Bridges the silver and gold options for builds that don't commit to one tone. Often called a champagne or bi-colour configuration. Gunmetal — deep cool-grey with smoky finish A cooler tonal cousin of black PVD — deeper than steel, lighter than full black, with a faint warm-grey undertone. The most unusual finish; pairs distinctively with brushed-steel dials or earth-toned leather. 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. Pair it with our Nautilus Skeleton Dial 30.8mm for a skeleton build, or any other 30.8 mm dial designed for the NH35-family. Bracelet: Stainless steel Seikonaut integrated bracelet included with the case. Standard spring bar attachment at the lugs, so you can swap to a rubber or leather strap if you prefer. Not compatible: 7S26 and 7S36 movements use a different stem height and are not recommended for this case. What's Included Seikonaut 40mm case body in your selected finish (316L stainless steel) Sapphire crystal in your selected AR coating (clear or blue) Caseback in your selected configuration (solid or exhibition) Screw-down crown Stainless steel Seikonaut integrated bracelet Gaskets (pre-installed) Tools are not included. You will need a case opener, hand press, stem cutter, and movement holder for assembly. Designed by Nomods The Seikonaut is our take on the ultra-thin Nautilus-style case for Seiko mods. The 9.95 mm solid-caseback profile is the thinnest configuration we make, designed for builders who want the Nautilus integrated-lug silhouette in a dress-watch profile. The exhibition caseback adds 0.35 mm for skeleton builds where you want the movement visible. Every case is individually checked for brushing consistency, edge sharpness, and crystal seating before shipping. Complete Your Build Pair the Seikonaut 40mm case with these compatible components: NH35 Movement — the standard automatic movement for this case NH72 Skeleton Movement — for skeleton builds with the exhibition caseback Silver V2 Hands — Nautilus-compatible hands in brushed silver Browse Collection Nautilus Style Mod Parts — All Seiko Mod Cases — All Seiko Nautilus Mods Nomods is an independent brand specializing in Seiko-compatible watch modifications. We are not affiliated with, endorsed by, or connected to Seiko, Rolex, Audemars Piguet, Patek Philippe, or any other watch brand mentioned on this site. 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