NH38 vs NH35

Side by side, from the spec sheets

The NH series is sold with specification sheets and drawings published by Time Module (TMI). Everything in this table is transcribed from the NH35A and NH38A sheets:

What the NH38 takes away: the date, and a crown position

The NH35A is described on its sheet as “Three Hands with Date,” with “date display with quick date correction,” and its dial drawings offer the date window at 3, 6 or 9 o’clock. The NH38A is “Three Hands” — no date at all.

The less obvious consequence is at the crown. TMI’s technical guide for the NH3 series gives the NH35A two pull-out clicks — the first sets the date, the second the time — and gives the NH38A only one, for the time. That matters to anyone building a no-date watch. Put a date-less dial on an NH35 and the date-setting click is still there, turning a date wheel hidden under the dial; the crown has a position that does nothing you can see. The NH38 removes the problem at the source, which is most of the reason it exists.

What the NH38 adds: the open heart

The NH38A’s cover drawing shows the balance visible through the dial on the 9 o’clock side, and TMI’s 2026–27 catalogue lists it as “3 hands, open-heart.” The dial drawing marks the aperture as a circle of up to 10.00 mm across, centered about 6.83 mm toward 9 o’clock and 0.56 mm below the center of the dial. The practical meaning: the NH38 is built for dials that show the balance wheel working, which is a look people search for on its own terms.

What stays the same, which is nearly everything

The base movement is shared. Diameter, height, jewel count, frequency, reserve, hacking and bidirectional automatic winding are identical on the two sheets, and both list the same antimagnetic rating of at least 4,800 A/m. The technical guide says the NH3 series is “fully wound up by turning the crown minimum 55 times,” and that applies to both.

For a modder, the useful part is the hardware. The two sheets give the same hand-hole diameters, the same hand-height options and the same stem part number, and their dial drawings put the dial feet in the same positions. That last point is our reading of the drawings rather than a sentence TMI prints, but it is what the coordinates say: a dial made for one sits on the other’s feet. A dial with a date window would, of course, frame nothing on an NH38.

Hand sizes, since this is the question people search

Per the sheets, the NH35A’s hand holes are 1.506 mm for the hour hand, 0.891 mm for the minute hand and 0.215 mm for the seconds hand, and the NH38A’s are identical. TMI’s catalogue groups the NH35, NH36, NH38 and the skeleton NH70 family in one hand-height column, consistent with the identical hole sizes on the sheets. The NH70 vs NH35 page covers the skeleton variant.

The two lines of fine print both sheets carry

Under casing, both the NH35A and NH38A sheets say a screw-type case back is required, and both say “Non-corresponding with Diver’s watch.” We read that as TMI declining to specify the movement for a certified diver’s-watch build; what a given finished watch is rated to is its maker’s claim, not the movement’s. Seiko’s own ISO 6425 divers use Seiko’s 4R and 6R calibers — see what those ratings certify.

Accuracy, and why it does not line up with Seiko’s figures

Both sheets give −20 to +40 seconds a day at 23 ± 2 °C, from a 20-second test. Seiko states its own 4R-series watches differently: +45 to −35 seconds a day, worn on the wrist between 5 and 35 °C, per its 4R instruction manual. Those are different tests under different conditions, so the numbers cannot be lined up to say which is tighter. What can be said is that the NH35 and NH38 carry the same figure, so accuracy is not a reason to choose between them.

On the Seiko side, the same manual lists a 4R38 as a three-hand caliber (hour, minute, second) at +45 to −35 seconds a day and approximately 41 hours. Seiko does not publish a statement that the 4R38 and NH38A are the same movement; the general relationship between the NH and 4R families is covered on NH35 vs 4R35.

Which one to choose

• ·Choose the NH35 if you want a date. It is the default for a reason, and the three window positions give a dial designer room.
• ·Choose the NH38 for a symmetrical no-date dial with no dead crown position, or for an open-heart dial that shows the balance.
• ·Choose neither if you want the full skeleton look — that is the NH70 family — or a day-date, which is the NH36.

Nothing to buy on this page, deliberately

This site only puts a buy button on a finished watch it has verified in its product registry, and none of those runs an NH38 or a loose NH35. If what you actually want is a finished Seiko to build on, the best Seikos to mod ranks the platforms, and the full NH35 specification is on the NH35 reference.

What we are not claiming

We have not handled, opened or timed either movement. Every figure is transcribed from TMI’s sheets, drawings and catalogue, with drawing units converted from hundredths of a millimeter. Where we have interpreted a drawing rather than quoted a statement — the dial-foot positions — the page says so.