If you've spent any time in the watch modding community, you've run into these two names more than any others: the Seiko NH35 and the Seiko NH36. Both are automatic movements manufactured by Seiko Instruments (TMI), and both have become the beating heart of the affordable custom watch scene. They're the direct successors to the legendary 7S26 found in classic Seiko divers, but with two upgrades modders love — hacking seconds and hand-winding.
So what actually separates them, and which one is easier to build a custom watch around? Let's break it down.
The Core Difference: Date-Only vs. Day-Date
The NH35 and NH36 share the same base architecture — 24 jewels, a 21,600 bph beat rate, roughly 41 hours of power reserve, and identical dimensions (28.4mm diameter, 5.32mm height). Mechanically, they're nearly twins. The difference that matters most to a modder is what shows up on the dial:
- NH35: Displays only the date in a single aperture at the 3 o'clock position.
- NH36: Displays both the day of the week and the date, with two windows near 3 o'clock.
That single difference cascades into everything else about how easy — or complicated — each movement is to customize.
Moddability: Why the NH35 Is the Beginner's Favorite
The NH35 is, hands down, the more forgiving movement to mod. Because it only needs one aperture cut into the dial, modders have far more creative freedom with dial layout, indices, and printing. There's no day wheel language to worry about, no second window to align, and less risk of a misaligned date wheel throwing off the whole look of the finished watch.
This is why the majority of DIY Seiko mod kits, aftermarket SKX-style dials, and beginner-friendly builds are designed around the NH35. If you're modding your first watch, this is generally the easier — and more common — starting point.
Moddability: The NH36 Rewards More Experienced Builders
The NH36 isn't harder to mod because the movement itself is more delicate — it's harder because the dial design gets more complicated. You now have two apertures to align correctly, and the day wheel typically only comes in a couple of pre-set language pairings (commonly English/Spanish or English/another language), which limits your options unless you're sourcing a specialty wheel.
That said, a well-executed day-date mod has a distinct payoff: it gives a custom watch a more "grown-up," complication-rich look that closely mirrors classic Seiko 5 or vintage dress-watch designs. For modders who've already built a few NH35-based watches and want to level up, the NH36 is the natural next challenge.
Specific Components Worth Modding
Whichever movement you're working with, most of the character of a modded watch comes down to a handful of swappable parts. Here are the components worth focusing on, and where to look for drop-in replacements:
1. Dials
The single biggest visual change you can make. NH35 builds give you a blank canvas — sunburst, matte, textured, or lumed dials all work well with just one date cutout. NH36 dials require a day-date cutout, so double-check compatibility before ordering.
2. Hands
Swapping hour, minute, and second hands (sword, pencil, snowflake, or lume-filled styles) can completely change the personality of the watch without touching the movement itself.
3. Chapter Rings
The printed or engraved ring around the dial edge is an easy, high-impact upgrade — useful for adding a minute track, a different color accent, or a vintage look.
4. Crowns & Crown Tubes
A signed or oversized crown is a small detail that makes a mod feel more "finished" and premium, and it's one of the simplest parts to swap.
5. Bezels & Bezel Inserts
For diver-style builds, a new bezel insert (ceramic, aluminum, or ceramic-look) is one of the most popular ways to change a watch's whole vibe — think blue/black "Batman" or full lume options.
6. Day Wheels (NH36 only)
If you're working with an NH36, sourcing an alternate day wheel is one of the more advanced but rewarding upgrades, letting you match the day display language or color to your build.
*Image used were generated with tools for purposes of accompanying the article only. It does not reflect actual products.