How to Build a Seiko Mod: The Complete Beginner's Guide

A first Seiko mod is more approachable than most people expect. A basic dial and hands swap is the most common entry point. It requires no watchmaking experience, just patience and a small set of dedicated tools. Budget £100–£400 (roughly $100–$500 USD) for a complete build. Movement, case, dial, and hands typically account for most of that cost, with the rest going to tools if you don't own them yet.

The four tools worth buying before you start: a spring bar tool for removing and fitting straps and bracelets, a movement holder or movement cushion to keep the movement stable while you work, a hand-setting tool or hand press for fitting the hour, minute, and seconds hands without bending them, and a caseback wrench or opener to close the case back securely. A basic starter set covering all four costs around £30–£50.

Parts can be sourced from dedicated watch mod suppliers. The Assemble Watches builder links directly to compatible parts so you can order without having to cross-reference compatibility yourself.

The movement is the engine inside the watch. It determines which dials and hands are compatible, so it makes sense to lock it in first. For most Seiko mod builds, the choice is between the NH35 and the NH36.

The NH35 is the more common choice. It's an automatic movement with a date display at 3 o'clock, 24 jewels, 21,600 bph frequency, a ~41-hour power reserve, and both hacking and hand-winding. TMI lists the movement at 27.40mm outside diameter, or 29.36mm with the dial holding spacer, and 5.32mm height. It's been the default movement for aftermarket Seiko builds for years, which means the parts ecosystem around it is vast. Typical price: £20–£35.

The NH36 is essentially the same movement with one difference: it adds a day display alongside the date. TMI lists the NH35A and NH36A with the same published height and footprint, so the practical difference is usually the dial. Most aftermarket dials specify which movement they're cut for, and NH36 dials need a day-date aperture. The NH36 runs around £30–£45 and is worth choosing if the day display matters to you. If you're unsure, start with the NH35 because more dials are cut for it.

Read the full NH35 movement guide →

The case is the outer shell of the watch. For NH35 and NH36 builds, the starting point for most modders is the SKX-style case: a sports case with a 22mm lug width that was designed around the same general footprint as the original SKX007. The NH35 drops straight into most cases built for this ecosystem.

When evaluating cases, check three things: lug width (the measurement between the lugs where the bracelet or strap attaches; 20mm and 22mm are the most common), case diameter (typically 40–45mm for SKX-style builds), and internal depth. The total stack of movement, dial, hands, and crystal needs to fit within the case without the crystal pressing against the hands or dial. Cases designed for the NH35 usually have adequate depth, but very slim dress-style cases may not.

Water resistance ratings on aftermarket cases vary widely. If water resistance matters to your build, look for cases with screw-down crowns and sealed casebacks, and have the watch pressure-tested by a watchmaker after assembly.

Dials have two small metal tabs on the underside called dial feet, which locate the dial on the movement. The NH35 uses Seiko-standard dial feet spacing, shared with the SKX007. Any dial listed as NH35-compatible or SKX-compatible should fit. If your dial has a date window, confirm it's cut for your movement. NH35 dials have a date aperture sized for the NH35 date wheel; NH36 dials need a day-date aperture.

Hands are matched to the movement's pinions: the NH35 uses standard Seiko NH hand sizes, roughly 1.50mm for the hour hand, 0.90mm for the minute hand, and 0.20mm for the seconds hand. Any hand set listed as NH35 or SKX-compatible should fit. The practical choice is aesthetic: pick hands that suit the dial's style. Sword hands read differently against a textured sunburst dial than against a clean matte one, and lume plots on the hands need to pair with lume on the dial if you want even glow at night.

The bezel is the ring that sits on top of the case around the crystal. Most SKX-style mod cases use a standard snap-on bezel that accepts aftermarket inserts in ceramic, aluminium, or sapphire. Unidirectional dive bezels are the most common. The bezel insert is a separate purchase from the bezel itself, so check that both are compatible with your case before ordering.

Crystal diameter is determined by your case, not your movement. For SKX-style cases this typically falls between 29.5mm and 30.5mm. Flat sapphire crystals are the default for a clean look; domed crystals add a vintage feel; AR-coated options reduce glare. Some crystals use a tension ring to hold them in place, others are adhesive. Your case spec will tell you which type to use.

Bracelets and straps attach at the lug width. For a 22mm case, any 22mm strap or bracelet fits. Oyster-style and jubilee-style bracelets are popular for dive builds; leather and NATO straps work for casual or dress styles. The lug width is fixed by the case, so the main constraint is making sure the spring bar holes in your chosen strap are cut for the right lug width.

Most Seiko mod parts come from a handful of specialist suppliers who stock movements, cases, dials, hands, crystals, and bezels specifically for aftermarket builds. These suppliers are distinct from general watch retailers. They exist to serve modders, so they label parts with movement compatibility, list dimensions accurately, and understand the questions you'll have.

For movements, buy from a supplier who explicitly sells genuine Seiko SII or TMI movements. Counterfeit NH35s exist on grey-market platforms; the tell is an unusually low price and no explicit confirmation of the brand. A genuine NH35 from a reputable mod supplier runs £20–£35. Anything significantly cheaper is worth being cautious about.

For dials and cases, the market is broader. Independent dial makers, small batch producers, and community creators all sell through their own sites and platforms. Prices and quality vary more here than with movements. The Assemble Watches builder links directly to suppliers for each part in the catalogue, so you can move from planning to purchasing without having to cross-reference stock separately.

With parts in hand, assembly follows a consistent order. Start with the movement: seat it in the movement holder, fit the stem and crown if they aren't pre-installed, and set the date using the crown before installing the dial. Once the dial is aligned and the feet are seated securely in the movement's dial feet holes, you can fit the hands.

Hand setting is the step that catches most beginners. Use a hand press or hand-setting tool to push each hand down on its tube: the hour hand first, then the minute hand, then the seconds hand. Work slowly. The hands need to sit flat and parallel to the dial; a hand that's even slightly angled will catch on the crystal or another hand as the movement runs. Check clearance by rotating the hands manually through a full cycle before fitting the movement in the case.

Once the dial and hands are set, lower the movement into the case and fit the caseback. Use your caseback wrench to tighten it. Hand-tight is not enough; the caseback needs to be seated fully against the gasket to maintain any water resistance. If your build is for regular wear or water exposure, take it to a watchmaker for pressure testing before getting it wet.

For a first build, having a local watchmaker check your work is genuinely worthwhile. Most watch repair shops will do a quick inspection inexpensively, and it's much easier to fix a loose stem tube or a crooked hand before the watch goes on your wrist than after.