The Seiko NH38 Explained: The No-Date, Open-Heart Workhorse

The NH38 is the NH35's cleaner sibling—same reliability, no date window, and one feature that changes everything: an open balance wheel at 9 o'clock.

Seiko's NH35 family has become the default choice for microbrands and modders who want affordable, reliable automatics. The NH38 is the no-date variant with a twist: it's designed specifically for open-heart dials, letting you see the balance wheel beating through a cutout on the dial side.

For buyers and builders who want dial symmetry, visible mechanical action, or both—this is the movement that enables it.

Here's everything you need to know.

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What Is the NH38?

The NH38 is an automatic movement manufactured by Time Module Inc. (TMI), a brand of Seiko Instruments Inc. (SII) that produces movements for third-party use. It's part of the NH35 family—a series of workhorse calibres that share the same fundamental architecture but differ in complications and features.

The NH38 has two defining characteristics:

No date. No date wheel, no date window, no quickset mechanism. The dial remains uninterrupted.

Open balance wheel. The balance wheel is visible at 9 o'clock on the dial side, designed for open-heart dial configurations. This isn't an afterthought—it's engineered into the movement's architecture.

Everything else mirrors the NH35. Same dimensions, same mounting points, same proven reliability.

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The Specs

The specs won't excite anyone chasing high-beat movements or 80-hour power reserves. That's not the point. The NH38 is a known quantity: robust, repairable, and cheap enough that movement cost doesn't dominate a watch's price.

The 41-hour power reserve means a full wind carries you through the weekend. Hacking (seconds hand stops when you pull the crown) allows precise time-setting. Hand-winding lets you top up the mainspring without wearing the watch.

At 5.32mm thick, the NH38 is slim enough for dress watch applications while remaining robust enough for daily wear.

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The Open Balance Wheel: Why It Matters

The NH38's open balance wheel is its distinguishing feature—and the reason it exists as a separate calibre rather than simply being an NH35 with the date mechanism removed.

What you're seeing:

The balance wheel is the regulating organ of the movement. It oscillates back and forth at 21,600 beats per hour—six times per second—controlled by the hairspring. This oscillation is what divides time into measurable units.

Through an open-heart dial, you watch this happen in real time. The balance wheel swings, the escapement releases, the gear train advances.

The design implication:

Open-heart dials require careful positioning. The NH38 places the balance wheel at 9 o'clock on the dial side, allowing designers to create symmetrical layouts with the aperture balancing against the small seconds or other elements.

This isn't possible with the NH35. The date mechanism occupies space that would interfere with open-heart configurations. The NH38 removes that obstacle by design.

Who cares about this:

Buyers drawn to visible mechanics. The open heart is polarising—some find it gimmicky, others find it captivating. If watching a movement breathe appeals to you, the NH38 enables it at an accessible price point.

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NH38 vs NH35 vs NH36: The Family Breakdown

The NH35 family shares DNA. The differences are in complications and features:

Note: The NH36 displays both day and date in a combined window at 3 o'clock, not as separate complications.

Internally, these movements share the same fundamental architecture. Same gear train, same escapement, same power source. The NH36 adds day/date wheels and associated mechanism; the NH38 removes calendar components and exposes the balance wheel.

Why does this matter?

If you're choosing between them, the decision is functional: Do you want a date? Do you want visible mechanics? The movements themselves are equally reliable.

For modders, the NH38's lack of date mechanism means no date wheel alignment issues, no ghost date position when setting time, and no need to avoid the danger zone (roughly 9pm–3am) when adjusting. Combined with the open balance wheel, it's the natural choice for open-heart builds.

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Why No-Date Matters

Beyond the open balance wheel, the no-date configuration has its own advantages.

Dial symmetry:

The date window breaks visual balance. On a three-hand watch, the hour, minute, and seconds hands create natural harmony. A date aperture at 3 o'clock disrupts that. Without a date, indices can be symmetrical, and the dial reads as a unified design.

Nothing to set:

Pick up a watch that's been sitting for a week, wind it, set the time, done. No scrolling through 31 days because you last wore it mid-month. No risk of damaging the quickset mechanism by changing the date at the wrong time.

Mechanical simplicity:

The date mechanism adds components that can fail. Quickset mechanisms can strip. Date wheels can slip. Without these components, the movement has fewer potential failure points.

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Where You'll Find the NH38

The NH38 appears in specific categories:

Open-heart watches:

The primary use case. Brands building watches with visible balance wheels choose the NH38 because it's designed for exactly this application. The 9 o'clock position allows natural dial layouts with the aperture as a deliberate design element.

Microbrands prioritising clean aesthetics:

Beyond open-heart designs, brands building field watches, dress pieces, and minimalist designs choose the NH38 when dial symmetry matters and date complications aren't needed.

The modding community:

Seiko modding is a hobby unto itself, and the NH38 is a staple for open-heart builds. The movement's dimensional compatibility with NH35 cases makes it a drop-in option for custom pieces.

Entry-level dress watches:

At the affordable end of the dress watch market, the NH38 enables clean, sophisticated dials without the cost of Swiss movements.

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The Case for No-Date

Zero setting hassle:

Date mechanisms require attention. Set the date in the wrong time window and you risk damaging the quickset mechanism. Forget to adjust after months with 30 days and you're off until you notice. None of this applies to no-date watches.

One less thing to break:

Fewer components means fewer failure points. In practice, NH35 date mechanisms are robust enough that this rarely matters—but the engineering principle holds.

Better dial options:

Watch designers working with no-date movements have more freedom. Symmetrical indices, uninterrupted dial textures, and balanced layouts become possible.

Vintage compatibility:

Many vintage watch designs predate ubiquitous date displays. If you're drawn to mid-century aesthetics, no-date movements align with historical accuracy.

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The Case Against

If you use the date, you'll miss it.

Some people glance at their watch for the date constantly. If that's you, no design purity compensates for fumbling for your phone to check what day it is.

Open-heart isn't for everyone:

The visible balance wheel polarises. Some find it adds interest and mechanical honesty. Others find it disrupts dial aesthetics or looks like a gimmick. Know which camp you're in before committing.

Know your habits before choosing:

Wear your current watch for a week and notice how often you check the date. If the answer is "constantly," the no-date life isn't for you.

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Accuracy and Reliability

The NH38 inherits the NH35 family's reputation.

Accuracy specification: -20/+40 seconds per day. This is Seiko's official tolerance—the range within which they consider a movement acceptable.

Real-world accuracy: Most NH38 movements run better than spec. Typical observed accuracy is -10/+20 seconds per day, with many units performing within ±10 seconds. Individual movements vary.

For context: a movement running +10 seconds per day gains about a minute per week. You'll adjust it occasionally. This is normal for unregulated movements at this price point.

Can accuracy be improved?

Yes. A competent watchmaker can regulate an NH38 to run within ±5 seconds daily. The movement responds well to adjustment.

Longevity:

The NH35 family has a proven track record. Movements run for years with minimal service. When service is needed, any watchmaker can work on them—parts are available, procedures are documented, costs are reasonable.

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NH38 vs Alternatives

Miyota 9039

The most direct competitor for no-date applications (though without the open balance wheel):

The differences that matter:

The 9039 runs at a higher beat rate (28,800 vs 21,600 bph), creating a smoother seconds hand sweep—8 steps per second instead of 6. It also has a tighter accuracy specification.

But the 9039 lacks the open balance wheel.

Which to choose?

For open-heart builds: NH38, no contest.

For closed dials prioritising sweep smoothness: Miyota 9039.

For pure price sensitivity: NH38 edges it.

Swiss Alternatives

Entry-level Swiss automatics (Sellita SW200, ETA 2824) typically include date complications. No-date Swiss configurations exist but are less common—many brands simply use date movements with dials that cover the window, leaving the date wheel inside.

True no-date Swiss movements command premiums. By the time you're paying Swiss prices, you're in a different market segment where movement cost isn't the primary constraint.

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The Verdict

The NH38 is two things: the NH35 without a date, and a movement purpose-built for open-heart dials. If either appeals to you, this is the default choice at accessible prices.

Choose the NH38 if:

• You want an open-heart dial with visible balance wheel
• Dial symmetry matters to you
• You rarely use the date function
• You're building or modding and want maximum dial flexibility
• You want proven reliability without Swiss pricing

Look elsewhere if:

• You check the date frequently
• Higher beat rate matters more than visible mechanics (consider Miyota 9039)
• You specifically want Swiss movement provenance
• Open-heart aesthetics don't appeal to you

The NH38 isn't just an NH35 with parts removed. It's a movement designed for specific applications—open-heart displays and clean dials—at a price that makes mechanical watchmaking accessible. That's the point.