Seiko 4R35 and 4R36

The 4R35 and 4R36 are the automatic calibers Seiko puts in its cheapest mechanical watches. The 4R35 shows a date, the 4R36 a day and a date, and both run at 21,600 vibrations per hour with more than approximately 41 hours of reserve. They hack, they hand wind, and they are inside a very large number of the watches on Seiko's own price lists.

They are also, in every respect a buyer can verify, the same design that Time Module sells to other watch companies as the NH35A and NH36A. This page is mostly about what that does and does not mean.

The specification Seiko publishes

The last row is the one people find surprising. Seiko lists power reserve, jewels, magnetic resistance and an operating temperature range for these calibers, and prints no accuracy figure at all. Its current product pages behave the same way: a 6R35 Prospex page states a precision of plus 25 to minus 15 seconds a day, while a 5 Sports page with a 4R36 in it simply omits the line.

Same movement, different label: what actually differs

Start with what does not differ. The plate size, the height, the beat rate, the reserve, the winding system, the shock protection, the calendar works and the hacking mechanism are common to both. A watchmaker opening a 4R36 and an NH36A side by side is looking at the same movement, and parts behave accordingly.

The differences that survive checking are these:

• The channel. Seiko fits 4R calibers to watches it sells under its own names. Time Module sells NH calibers to anybody who will buy a batch. The number on the movement tells you which door it came through.
• The published jewel count of the date-only version. Seiko's booklet says 23 jewels for the 4R35. Time Module says 24 for the NH35A. The day and date versions are both published at 24.
• The documentation. Time Module publishes a technical guide with a static rate of minus 20 to plus 40 seconds a day, a lift angle, a positional tolerance and a full parts catalogue, because its customers need those things to build watches. Seiko publishes none of that for the 4R, because its customers are wearers.
• The options. Time Module offers outside customers a catalogue of stem lengths, hand heights, date wheel colors and reference codes. Seiko orders what its own designs need.
• The finish you can see. On a Seiko with a display back the oscillating weight is marked and decorated to Seiko's taste. Third party brands get a plainer weight, or add their own.

One small piece of evidence about how closely the two are related: Seiko's own global instruction library serves the owner's manuals for this family under the names NH35 and NH36. There is no separate 4R35 or 4R36 booklet in it.

What it replaced, and what changed

The 4R35 and 4R36 took over from the 7S35 and 7S26, and Seiko documents both generations in a single booklet, which makes the comparison easy. The 7S26 is listed as automatic winding, 21 jewels. The 4R36 is listed as automatic winding with a manual winding mechanism, 24 jewels. Same reserve, same beat rate, same magnetic resistance.

So the upgrade was two features rather than a new engine: you can now wind the watch by hand instead of shaking it, and you can stop the seconds hand to set it against a time signal. Both matter more day to day than any accuracy number. The SKX007 ran the 7S26 and the current Seiko 5 Sports runs the 4R36, which is why the newer watch is easier to live with and, in other respects, less watch.

Rate: the figure Seiko does not print

Because Seiko publishes no daily rate for the 4R, the only maker's figure available for this design is Time Module's: minus 20 to plus 40 seconds a day, measured over three positions between 10 and 60 minutes after a full wind, with a permitted spread of under 60 seconds across four positions.

Set against a chronometer criterion of minus 4 to plus 6 seconds a day, that is a band six times wider. It is also, as an acceptance window, close to meaningless as a prediction: factories print the edge of the window, and most movements land nowhere near it. A 4R36 running within ten seconds a day is entirely ordinary, and so is one running at plus thirty. Both are inside specification, which is exactly why a specification this wide tells an owner so little. The guide to accuracy and chronometer standards explains what the tighter numbers actually buy.

Two practical notes. Position matters more than the movement on a caliber with this much permitted positional error, so where you rest the watch overnight will change your weekly result. And these movements are not magnetically shielded beyond the stated 4,800 A/m, so a sudden gain of a minute a day is usually magnetism rather than damage.

Servicing, parts and cost

Any independent watchmaker who works on Seiko can work on a 4R. The parts are the NH parts, and Time Module publishes the catalogue, so identification is straightforward even when supply is not.

The economics are the same as for the NH35 and just as blunt. A full overhaul at an independent runs roughly $250 to $500, while a replacement movement costs a small fraction of that. Figures are indicative US dollars before sales tax, and they move with country and labor rates. For a watch bought for $275 to $450, a movement swap is the rational repair, and a good watchmaker will tell you so before you ask.

Service is still worth it when the movement only needs regulating, demagnetizing or a new mainspring, when the dial and hands will not transfer cleanly to a stock movement, or when the watch is a limited edition where an original movement is part of what you own. Inside the warranty period, send it to Seiko and let them decide.

Known weaknesses

• Wide positional variation. Permitted at under 60 seconds across four positions on the equivalent NH specification, and visible on poor examples.
• Coarse regulation. An index regulator and a movable stud carrier, no fine adjuster. Adjustable, but not quickly.
• A long date hazard window. The manuals say not to change the date between 9:00 p.m. and 4:00 a.m., a seven hour exclusion that is easy to forget on a day and date version.
• Rough hand winding. The keyless works are made to a price and feel it. Normal, not a fault.
• No published rate. You are buying an accuracy specification that the maker declines to state, which is not a defect but is worth knowing.

Which watches use it, and whether to care

The 4R36 runs most of the current 5 Sports range and a large part of the entry level Prospex divers. The 4R35 turns up across Presage and other date-only models. Between them they account for the majority of Seiko's mechanical output by volume, which makes this quietly one of the most produced automatic calibers in the world.

If you are choosing between a Seiko with a 4R36 and an independent brand's watch with an NH36, the movement is not the deciding factor. Compare the case, the crystal, the bracelet, the water resistance rating and who honors the warranty. The guide to microbrands and independents covers why supplied calibers dominate that comparison, and the three-way NH35 against Miyota 9015 against Sellita SW200 comparison puts the alternatives side by side. If you want a longer reserve and a rate Seiko is willing to publish, that is the 6R35, in watches costing several times as much. If you want a thinner watch with a faster balance, that is the Miyota 9015.

Buy a 4R watch if you want a reliable everyday mechanical and would rather the money went into the case and bracelet. Do not buy one expecting precision, refinement of feel, or a movement worth restoring in thirty years.