Seiko Mod Cases

About

21 stainless steel mod cases across four platforms: Seikonaut / Nautilus Ultra-Thin (40 mm), Petrichor (37 mm), Royal Oak 41 mm , and Royal Oak 37 mm V2. Every case ships with a sapphire or domed sapphire crystal, screw-down crown, and water resistance from 50 m to 100 m. All cases accept Seiko NH35, NH36, NH70, NH71, and NH72 movements out of the box. Movement compatibility: Every case in this collection fits the full NH35 family (NH35, NH36, NH38, NH34 GMT) and NH70 skeleton family (NH70, NH71, NH72). No adapter or spacer needed. Dial fit: Seikonaut and Royal Oak 41 mm cases take 30.8 mm dials. Petrichor and Royal Oak 37 mm V2 cases take 28.5 mm dials. Check the dial size guide before ordering. Watch out: Dial size is the most common mistake in Seiko modding. A 28.5 mm dial will not fit a Seikonaut case, and a 30.8 mm dial will not fit a Royal Oak 37 mm. Always match the dial diameter to your case platform. How to Choose Your Case Style: The Seikonaut delivers a Nautilus-inspired silhouette with integrated lugs and a horizontally ribbed bezel. The Petrichor is a dressy 37 mm design with a domed sapphire crystal and slim 9.95 mm profile. The Royal Oak 41 mm offers the classic octagonal bezel at a sportier size with 100 m water resistance. The Royal Oak 37 mm V2 brings the same octagonal DNA into a more compact, wrist-friendly package at 9.95 mm thick. Size: The Petrichor and Royal Oak 37 mm V2 both wear at 37 mm and sit under a shirt cuff comfortably. The Seikonaut at 40 mm hits the modern sweet spot. The Royal Oak 41 mm wears larger at 41 × 51 mm with a 12.2 mm thickness — best for wrists 17 cm and above. Movement choice: If you plan a skeleton build with an NH70, NH71, or NH72, all four platforms work. For a solid-dial build with an NH35 or NH36, the same applies. The case does not restrict your movement choice. Crystal: Petrichor cases feature a domed sapphire for a vintage-inspired profile. Seikonaut and Royal Oak cases use flat sapphire crystals for a cleaner,