NH35 vs NH36: which Seiko movement for your mod?
Choosing the movement is the first technical decision for any Seiko Mod project. Two calibers dominate the modding market: the NH35 and NH36. Both are manufactured by Seiko Instruments (SII), distributed by TMI, and share the same base architecture. Yet they serve different purposes. This comparison details the concrete differences to help you decide.
Origins and shared architecture
The NH35 and NH36 belong to the 4R movement family. The NH35 derives from the 4R35 caliber, the NH36 from the 4R36. They share a 28.5 mm diameter, 5.32 mm thickness and the same mounting points. This means both movements fit the same cases without modification. The crown stem, gaskets, everything is interchangeable.
Power reserve is identical: 41 hours. Winding is bidirectional (the rotor charges in both rotation directions). Both offer hacking, the function that stops the seconds hand when you pull the crown to set the time precisely. It's a standard expected by any mechanical watch enthusiast.
The NH35: no-date caliber
The NH35 is the simpler version. No date window, no disc, no intermediate crown position. Two positions only: manual winding and time setting. For a first mod, this simplicity is an advantage. Fewer parts to align, less risk of error during assembly.
This movement suits clean dials with no opening. Minimalist designs, Explorer-type or field watch dials, dress watches: the NH35 is their natural partner. Dial symmetry is preserved, giving a cleaner look on the most understated dials.
In terms of accuracy, expect +/- 20 seconds per day from factory. A watchmaker can adjust regulation to +/- 10 seconds with basic tools. That's comparable to Swiss movements sold at three times the price.
The NH36: date and day
The NH36 adds two complications: date and day of the week. A window at 3 o'clock displays the date. On some dials, a second or combined window also shows the day. The crown gains three positions: winding, quick date/day setting (without advancing the hands), and time setting.
It's the most popular movement in Seiko Mod. The majority of dials from suppliers feature a date opening. The NH36 is therefore the default choice for most projects, especially divers, GMT homages and sport watches.
A technical point: the NH36 date disc comes in black or white. Parts suppliers also offer custom date discs (color, font). Replacing the stock date disc with one matching the dial creates a more cohesive result.
How to choose between them?
The rule is simple: look at your dial. If it has a date window, get an NH36. If it doesn't, get an NH35. Technically, an NH36 works with a dateless dial. The date disc spins behind the dial in the void. Not a mechanical problem, but an unnecessary complication adding noise thickness to the movement.
The reverse isn't possible: an NH35 in a date-window dial leaves an empty hole. No disc, no display.
Beyond the NH35/NH36 duo
For advanced modders, Seiko offers other calibers in the same family. The NH38 is a thinner NH35 variant, designed for dress cases. The NH34 adds a GMT complication with an independent 24-hour hand. The NH70 is a skeleton movement for open-worked dials, a rare but spectacular choice.
The Miyota 9039 (by Citizen) is an NH35 alternative with better base accuracy (+/- 10 sec/day). But Miyota parts aren't compatible with the Seiko Mod ecosystem. If you stay within the SII/TMI world, the NH35/NH36 duo covers 90% of projects.
Price and availability
The NH35 costs 25 to 45 euros depending on the supplier. The NH36 between 28 and 50 euros. The price difference is marginal. Both are available from all parts suppliers: Namoki, Crystal Times, DLW Watches, AliExpress (verified sellers). Check our supplier guide for reliable sources.
One tip: always order a new movement, not one disassembled from a Seiko 5. New TMI movements are factory-tested and shipped with stem and rotor. A used movement may have invisible wear marks that show up after a few months.
The movement is the heart of your watch. Get the right one first try, and the rest of the build will follow smoothly. To understand how to assemble your first watch, check our beginner's guide.