So you've got a Seiko mod watch — maybe a GMT Bruce Wayne with the NH34, or a Datejust Wimbledon running the NH35 — and you've noticed it's running a few seconds fast or slow every day. You're not imagining it. Automatic movements do that. And the good news is, it's completely normal, completely fixable, and something every watch enthusiast eventually learns about.
That's what regulation is.
And once you understand it, you'll never look at your automatic watch the same way again.
What "Regulating" Actually Means
You're not redesigning the movement or doing anything irreversible. Regulating is simply adjusting how fast or slow the movement runs.
Inside the movement is a small lever called the regulator arm. Moving it in one direction speeds the watch up. Moving it the other way slows it down. That's the whole concept — a tiny physical adjustment that fine-tunes the beat rate of the balance wheel, which controls how the watch divides time.
Specifically on the NH35 and NH34:
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Moving the regulator toward "+" → makes the watch run faster
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Moving the regulator toward "–" → makes the watch run slower
Small movement, big effect. Which is exactly why patience and precision matter more than anything else here.
Why Do NH35 and NH34 Movements Need Regulation?
Here's something worth knowing upfront: the NH35 and NH34 are rated by Seiko at -20 to +40 seconds per day out of the box. That's the official tolerance range, and it's intentionally wide — these are reliable, mass-produced movements built for longevity, not chronometer-level precision from the factory.
In the real world, most NH35 and NH34 movements perform better than that rated range — typically somewhere between -5 and +15 seconds per day. But "better than spec" isn't the same as "as accurate as it could be."
A properly regulated NH35 or NH34 can realistically hit ±5 seconds per day or better — accuracy that rivals movements costing significantly more. The potential is already inside the movement. Regulation just unlocks it.
What You'll Need
You don't need a full watchmaker's bench to regulate your watch. At minimum:
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Caseback opener — the right tool for your specific caseback style (screw-down or snap-back)
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Fine adjustment tool — a toothpick is safer and recommended for beginners; a small non-magnetic screwdriver gives more precision but carries more risk
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Time reference — a phone synced to an atomic clock works fine
Optional but highly recommended:
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Timegrapher — gives precise readings in seconds/day, amplitude, and beat error. Takes the guesswork out of the process entirely.
Step-by-Step: How to Regulate Your NH35 or NH34
Step 1: Measure Your Watch First
Before touching anything, establish a baseline. You need to know how your watch is actually running before you adjust it.
Compare your watch to an accurate time source over 24 hours. Write down the difference.
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+25 seconds/day → running fast
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−20 seconds/day → running slow
If you have a timegrapher, even better — it gives you the exact rate in seconds per day without waiting.
Step 2: Open the Caseback
Use the correct caseback tool for your watch. Take your time here — a slipped tool scratches the case, and that's a mistake you can't undo.
Before opening, make sure the watch is clean and dust-free. Work in a dry environment. The movement is now exposed and even a small particle of dust can cause problems if it settles in the wrong place.
Step 3: Locate the Regulator
With the caseback open, you'll see the movement. Look for the balance assembly — it's the component oscillating back and forth, the visible heartbeat of the watch.
On the NH35 and NH34, near the balance assembly you'll find two levers:
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Regulator arm — marked with + and –. This is the one you adjust.
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Beat error lever — nearby, but DO NOT touch this unless you know exactly what you're doing. Beat adjustment is a different process entirely and getting it wrong causes more problems than it solves.
👉 Only adjust the lever clearly marked + / –.
Step 4: Make Tiny Adjustments
This is the most critical step — and the one where most people go wrong by moving too much.
Move the regulator arm very slightly. Think fractions of a millimetre. The NH35 and NH34 are sensitive: even a tiny movement of the regulator arm can change the rate by 10–30 seconds per day. Less is more, always.
Direction guide:
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Watch running fast → move toward "–"
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Watch running slow → move toward "+"
Tool choice:
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Toothpick — safer, less risk of slipping or damaging the movement
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Fine non-magnetic screwdriver — more precise, but higher risk if your hand isn't steady
Avoid magnetic tools. Magnetisation can cause a movement to gain minutes per day — a problem that requires a demagnetiser to fix and is much harder to deal with than a simple rate adjustment.
Step 5: Recheck Accuracy
After your adjustment:
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Close the caseback — you don't need to fully tighten it yet
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Wear the watch or let it run for several hours
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Measure the rate again against your time reference
If it's closer to where you want it, great. If not, repeat the process with another small adjustment. This is iterative — don't expect to nail it on the first attempt.
Give it time. Movements don't stabilise instantly after an adjustment. Wait at least a few hours — ideally a full day — before measuring again and making further changes.
Tips for Better Accuracy
Regulate in Real Conditions
Mechanical watches behave differently depending on position. A movement that runs perfectly flat on a table will run differently on a moving wrist. If possible, regulate based on how you actually wear the watch — not just how it performs sitting still.
For the most precise results, watchmakers test across six positions (dial up, dial down, crown up, crown down, crown left, crown right) and look for consistency across all of them. For most everyday wearers, regulating based on typical wrist-wear position is sufficient.
Don't Chase Perfection
The NH35 and NH34 are not chronometers, and that's fine. Realistic accuracy targets after regulation:
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±5 to ±10 seconds/day — excellent
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±10 to ±15 seconds/day — very good
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±20 seconds/day — normal, perfectly acceptable for daily wear
Chasing ±0 seconds/day on a mass-produced automatic is possible but time-consuming and usually unnecessary. Set a reasonable goal and stop when you get there.
Let It Settle
After each adjustment, give the movement time to stabilize before measuring again. A few hours minimum, a full day ideally. Measuring too soon after an adjustment gives you inaccurate data and leads to over-correction.
Special Note for NH34 (GMT) Owners
Good news if you're regulating a GMT Batman or any other NH34-powered build: the regulation process is identical to the NH35.
The GMT complication — the additional hand and gearing that tracks a second time zone — does not affect regulation directly. It's independent of the balance assembly and regulator arm. You adjust the NH34 exactly the same way you'd adjust an NH35. Same lever, same process, same patience.
Common Mistakes to Avoid
1. Moving the wrong lever The beat error lever sits close to the regulator arm. Adjusting the wrong one doesn't regulate the rate — it changes the beat error, which is a separate (and more complex) issue. Always confirm you're on the + / – marked lever before touching anything.
2. Over-adjusting The single most common mistake. One large movement of the regulator arm can throw the rate off by 30+ seconds per day in the wrong direction. Always go tiny. Repeat small adjustments rather than making one big one.
3. Magnetising the movement Using magnetic tools near the movement can magnetise the hairspring, causing dramatic time gain — sometimes minutes per day. Use non-magnetic tools wherever possible, and keep phones and magnetic clasps away from the open movement.
4. Dust and moisture Don't leave the caseback open longer than necessary. Work in a dry, clean environment. A particle of dust in the wrong place can cause inconsistent timekeeping that has nothing to do with the regulator setting.
Advanced: Reading a Timegrapher (Optional)
If you want to go deeper, a timegrapher is the tool that makes regulation significantly easier and more precise. It listens to the movement's tick and gives you:
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Rate — seconds per day (what you're adjusting)
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Amplitude — the arc of the balance wheel's swing, in degrees
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Beat error — the difference between the tick and the tock
For regulation purposes, focus primarily on rate. The other readings are useful context but don't require adjustment unless you're doing more advanced work.
Healthy targets on a regulated NH35/NH34:
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Rate: close to 0 s/day
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Amplitude: 250–300° (a healthy, well-wound movement)
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Beat error: under 1.0 ms (leave this alone unless you're trained in beat adjustment)
Your Watches, In Context
GMT Black Explorer II — NH34
The GMT Explorer II tracks two time zones simultaneously, which means daily rate accuracy matters more here than on a simpler build. A watch running +15 seconds per day compounds across both time zones — regulation brings that under control cleanly.
Datejust Twotone Arabic Silver — NH35
The everyday watch in the collection. Worn five or six days a week, all day — which means a few seconds of daily drift adds up fast over a working week. A well-regulated NH35 here means Monday's time is still accurate by Friday.
Yacht Master Rose Gold Black — NH35
Built for active wear, which means the movement is constantly in motion. Positional accuracy matters more on this build than on a dress watch. Regulating across real wear conditions — not just flat on a table — gives the best results.
The Bottom Line
Regulating your NH35 or NH34 is one of the most rewarding things you can do as a watch owner — not because it's complicated, but because it's not. It's a small, precise adjustment that unlocks the full accuracy potential of a movement that's already capable of impressive results.
The NH35 and NH34 were built for exactly this kind of adjustment. They're forgiving, robust, and more beginner-friendly than most automatic movements at any price point. Whether you do it yourself with a toothpick and patience, or hand it to a local watchmaker for a professional regulation, the result is a watch that performs at its best — every day, on your wrist.
Learn more at: "Seiko Modding: The Deep Dive Guide — Parts, Assembly, Tools & More" Blog
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