How to Build a Custom Seiko Watch Step by Step
A clear, beginner-friendly guide to planning, sourcing parts, assembling, sealing, and testing your first custom Seiko mod.
Introduction
Building a custom Seiko mod is one of the most rewarding projects in modern watchmaking. This guide walks you through each step—from planning and parts selection to assembly, alignment, sealing, and testing—so you can create a clean, reliable timepiece with confidence.
Watch the Build in Action
Prefer to see the full process step by step? Watch our Seiko mod build video:
Before You Start: Plan & Check Compatibility
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Define your build concept
Diver, field, pilot, dress, or GMT? Keep a reference image or sketch to guide part choices (dial, hands, bezel, colors, lume).
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Pick the base case
Choose a Seiko or Seiko-compatible case that suits your concept (lug width 20/22 mm, crown position, thickness).
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Select the movement
Common choices: NH35 (date), NH36 (day-date), NH34 (GMT). Ensure case supports the function set.
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Verify dial & hand compatibility
Match dial diameter and feet to the movement/case; match hand hole sizes to the caliber’s hour/minute/seconds pinions.
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Crystal, bezel, and insert fit
Confirm crystal diameter/thickness and bezel/insert inner–outer diameters per your specific case model.
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Gaskets & water resistance
Have fresh gaskets (caseback, crown, crystal, bezel). Plan a pressure test if water resistance matters.
Need Help or Want Us to Build It?
At Rexx Timepieces, we design and assemble bespoke Seiko mods and guide you through parts selection, alignment, and sealing.