If you're looking at a custom or modded watch, the choice usually comes down to what's ticking inside it. Two names come up more than any other: the Seiko NH35 automatic and the Seiko VK63 Meca-Quartz chronograph. They power very different watches, and the right one depends entirely on how you want the watch to feel on your wrist.
Here's a straight comparison, with no jargon left unexplained.
What is the NH35?
The NH35 is a Japanese automatic movement made by Seiko's TMI division. “Automatic” means it winds itself from the natural motion of your wrist — there's no battery, and you never need to replace one. Leave it on your wrist through the day and it keeps running; set it down for a couple of days and it winds down, then springs back to life with a few shakes or a turn of the crown.
It's become the default movement for custom and modded watches for good reason: it's reliable, serviceable, hacks (the seconds hand stops when you pull the crown, so you can set the time precisely), and hand-winds. It's a true mechanical movement at an accessible price.
Best for: anyone who wants the character of a mechanical watch — the sweeping seconds hand, the self-winding rotor, the feeling of a small machine on your wrist.
What is the VK63?
The VK63 is a Meca-Quartz chronograph movement, also from Seiko. “Chronograph” means it has a built-in stopwatch — the pushers on the side of the case start, stop and reset the sub-dials on the face. “Meca-Quartz” is the clever part: a quartz motor keeps the time (so it's very accurate), but a mechanical module drives the chronograph. That combination gives you the best of both worlds — the crisp, instant snap of the chronograph hand flying back to zero, which pure quartz chronographs can't replicate.
It runs on a battery, so it's low-maintenance and holds excellent accuracy over the long run.
Best for: anyone who wants a sportier, racing-inspired look with working sub-dials, plus quartz accuracy and minimal upkeep.
Head to head
Feel. The NH35 has a smooth, sweeping seconds hand and the subtle rotor wobble of a mechanical watch. The VK63 ticks once per second like a quartz watch — but its chronograph hand snaps back with a satisfying mechanical action.
Function. The NH35 tells the time (and date). The VK63 tells the time and works as a stopwatch via its sub-dials.
Accuracy. The VK63 wins here — quartz timekeeping drifts only seconds per month. A mechanical NH35 typically runs within a range of about +/- 20–40 seconds a day, which is normal and expected for an automatic at this level.
Maintenance. The NH35 never needs a battery but benefits from a service every several years. The VK63 needs an occasional battery change but is otherwise hands-off.
The look. NH35 builds tend to be clean three-hand watches — dressy, diver, or everyday. VK63 builds are chronographs, so they carry sub-dials and pushers and lean sporty.
So which should you choose?
Pick the NH35 automatic if you want the soul of a mechanical watch — self-winding, battery-free, with that living, sweeping movement. It's the natural choice for a clean everyday or dress piece.
Pick the VK63 Meca-Quartz if you want a chronograph — the racing look, working sub-dials, a snappy reset, and set-and-forget accuracy.
There's no wrong answer. It's a question of whether you're drawn to a mechanical everyday watch or a sporty, accurate chronograph.
How COSO builds them
Every COSO watch is assembled by hand and built to order — nothing sits on a shelf. Whichever movement you choose, it's cased with a stainless steel body and a scratch-resistant sapphire crystal, then inspected before it ships. Every order includes free tracked worldwide shipping, a 1-year warranty and 30-day returns.
- Want the mechanical, self-winding feel? Explore the automatic lines — Montier, Navex, Tolaris and Valtor — all built on the NH35.
- Want a chronograph? The Torvex collection is built on the VK63 Meca-Quartz, with working sub-dials.
- Not sure yet, or want it your way? Build your own COSO and choose the dial, case and strap yourself.
Whichever way you go, you're getting a watch made for you — not one pulled off a production line.