Seiko Mod Cases

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, more modern look.

Beginner recommendation: If this is your first build, the Royal Oak 37 mm V2 in Silver at $149 is the easiest starting point — it accepts the widest range of 28.5 mm dials and the slim profile looks good on almost any wrist. Read the step-by-step build guide before you start.

Quick Comparison

Style	Size	Thickness	WR	Price
Seikonaut / Nautilus Ultra-Thin	40 mm	9.95 mm	50 m	$149
Petrichor	37 mm	Slim (domed sapphire)	50 m	$195
Royal Oak 41 mm	41 × 51 mm	12.2 mm	100 m	$110
Royal Oak 37 mm V2	37 × 48 mm	9.95 mm	100 m	$149

Frequently Asked Questions

Which cases fit 28.5 mm dials and which fit 30.8 mm dials?

The Petrichor and Royal Oak 37 mm V2 take 28.5 mm dials. The Seikonaut / Nautilus Ultra-Thin and Royal Oak 41 mm take 30.8 mm dials. This is the single most important spec to check before ordering — the wrong dial size will not physically fit. See our dial size guide for a full breakdown.

Do I need to buy a crystal separately?

No. Every case ships with a sapphire crystal already installed. Petrichor cases include a domed sapphire; all other cases include a flat sapphire. You do not need to source or press a crystal yourself.

Which case is best for a first-time builder?

The Royal Oak 37 mm V2 is the most forgiving for beginners. It uses widely available 28.5 mm dials, supports every NH movement in the Nomods lineup, and the 37 mm size works on most wrists. Pair it with an NH35 and a solid dial for the simplest possible first build. Our build guide walks through the full process step by step.

Can I use a skeleton movement in any of these cases?

Yes. All four case platforms — Seikonaut, Petrichor, Royal Oak 41 mm, and Royal Oak 37 mm V2 — accept the NH70, NH71, and NH72 skeleton movements. Pair a skeleton movement with a matching skeleton dial in the correct size (28.5 mm or 30.8 mm) for your case. See our NH72 skeleton guide for build ideas.

Complete Your Build

Seiko Mod Dials — skeleton and solid dials in 28.5 mm and 30.8 mm to match your case
Seiko Mod Hands — polished and brushed hand sets compatible with all NH movements
Seiko Mod Movements — NH35, NH36, NH72, and more for solid or skeleton builds
Nautilus-Style Parts — dials, hands, and bracelets matched to the Seikonaut platform
Royal Oak Parts — dials, hands, and bracelets matched to Royal Oak cases

Nomods is an independent brand. Our products are original designs inspired by iconic watch aesthetics. We are not affiliated with or endorsed by any luxury watch manufacturer. All brand names are used for reference purposes only.

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