Seiko · Seiko 4R38 open heart

Seiko Presage Cocktail Time Open Heart power reserve: inside the 4R38

The Seiko Presage Cocktail Time Open Heart models, including the SSA441 Grasshopper and SSA405 Blue Moon, run on Seiko's 4R38. From a full wind it runs about 41 hours, or 1.7 days.

Updated September 2026 · Based on manufacturer specs, published reviews and watchmaker/owner reports

Watches that use the Seiko 4R38

If your Seiko has a round window at 9 o'clock showing the balance wheel swinging, and no date, it very likely has a 4R38 inside. Today it's best known from the open-heart versions of the Presage Cocktail Time, but Seiko has used it since the early 2010s in older Presage models, ladies' automatics and a few Seiko 5 Sports pieces. Outside Seiko, the same open-heart design is sold to other brands as the NH38.

Specs at a glance

History and where it came from

Seiko's technical guide for the 4R38A says plainly that it was developed from the 4R36A, the day-date workhorse inside the Seiko 5 Sports and the Turtle. Seiko dropped the calendar, gold-plated the balance wheel and arranged things so the balance sits under a cutout in the dial at 9 o'clock. The same document covers the 4R39, which adds a 24-hour hand to the same open-heart base. Everything else, from the 27.4 mm footprint to the 24 jewels, the escapement and the 41-hour mainspring, is shared with the rest of the 4R family.

Reference sites date the 4R38 to around 2012. It first appeared in Presage dress watches, ladies' open-heart models and some older Seiko 5 Sports pieces. It got its biggest audience in the early 2020s, when Seiko added open-heart versions to the Presage Cocktail Time line: the Blue Moon and Midnight Stinger, and then the Blue Acapulco and Grasshopper.

The open heart is a real cutout, not a printed effect. Crystal and dial still protect the movement, but you can see the balance swinging six times a second, which is the main reason people buy it over a standard Cocktail Time with a 4R35.

Accuracy in the real world

Seiko rates the 4R38 at +45 to -35 seconds a day, worn on the wrist at normal temperatures. That's the same tolerance as the 4R35 and 4R36, which makes sense, because the balance and escapement are the same. Seiko's service guide sets tighter workshop limits: within ±20 s/day dial-up when fully wound, and within ±30 s/day in the vertical positions and after 24 hours of running down.

There's no reason to expect a Cocktail Time Open Heart to keep time differently from a regular Cocktail Time. Owners of 4R-powered Seikos commonly report something between a few seconds and 20 seconds a day fast, with some examples near zero and a few near the edge of the spec. Rates change with position and how fully wound the spring is, so a watch that sat on a dresser dial-up overnight can behave differently from one that was worn all day.

It's regulated with a conventional regulator arm. That's a quick, cheap job for a watchmaker.

Reliability and servicing

The 4R38 has the 4R36's reputation: simple, robust and forgiving. It uses Seiko's Diashock shock protection and Magic Lever winding, both proven over decades. There's no known failure specific to the open-heart version.

Servicing is the same story as other 4R Seikos. Seiko service centres will overhaul it, and many independent watchmakers can service it or swap in a replacement movement. One caveat: because the balance is on show, a swap has to keep that balance lined up with the dial cutout. A 4R35 or NH35 won't do. It needs another 4R38 or an NH38.

Quirks worth knowing

• No date. A stopped 4R38 only needs the time set. There's no date wheel to run through and no late-night danger zone to worry about.
• AM and PM don't matter. With no date or 24-hour hand, the watch has no idea whether it's morning or night. The 4R39 adds a 24-hour hand to fix that.
• Winding from stopped. Seiko's manual notes that a stopped watch won't start straight away when you wind it, because the spring has little torque at first. Keep winding clockwise until the seconds hand moves, or give the watch a gentle side-to-side swing.
• The open heart shows everything. You'll see the balance stop the moment the reserve runs out, which is a useful visual check. You'll also see the plated parts, which aren't finished to a high standard at this price.

How it compares

• Seiko 4R35 / 4R36: the same movement with a date (4R35) or day-date (4R36) and no open heart. Choose the standard Cocktail Time if you want a date window.
• Seiko 4R39: the same open-heart base with a 24-hour hand, used in the Style60's and Japanese Garden Presage models.
• Seiko 4R72: goes further than an open heart and skeletonises the whole movement, with the same 41-hour reserve.
• Seiko 6R5J: the upmarket open-heart Presage calibre, with a 72-hour reserve that survives a weekend in a drawer. It costs noticeably more.
• Seiko NH35: the unbranded date version of the same family, found in countless microbrand watches.

Living with the 41-hour reserve

Forty-one hours is short, and the open-heart Cocktail Time is a watch people tend to save for evenings and weekends. Take your SSA441 off at 11pm on Friday after dinner and, if it was fully wound, it will stop around 4pm on Sunday. If you only wore it for a few hours, it may already be stopped by Saturday night. You'll know, because the balance behind the cutout stops moving.

The fix is easy: give it a few dozen clockwise turns of the crown before you put it on, and don't count on a short evening's wear to top it up. If you rotate a Cocktail Time Open Heart with a daily watch, Watch Winder counts down the 41-hour reserve for each watch you own and reminds you before it stops. To check a specific weekend, try the "Will my watch stop?" calculator.

Verdict

The 4R38 is a 4R36 with its calendar removed and its balance put on show, and that's both its appeal and its limit. You get Seiko's proven entry-level automatic, with hacking, hand-winding and easy, cheap servicing, plus a bit of theatre on the dial. You also get the 4R's loose official tolerance and a 41-hour reserve that won't last a weekend. For a dress watch like the Cocktail Time Open Heart, which will spend days off the wrist, plan on winding it rather than relying on the rotor.

FAQ

How long does the Seiko Cocktail Time Open Heart run off the wrist?

About 41 hours from a full wind, roughly a day and three quarters. If it was only lightly worn before you took it off, it will stop sooner.

Does the Seiko Presage SSA441 hack and hand-wind?

Yes. The 4R38 in the SSA441 stops the seconds hand when you pull the crown out to set the time, and you can wind it by turning the crown clockwise.

How accurate is the Seiko Presage Cocktail Time Open Heart?

Seiko rates the 4R38 at +45 to -35 seconds a day on the wrist. Most examples run well inside that, often a few to 20 seconds a day fast, and a watchmaker can regulate it closer.

Why did my Seiko SSA405 stop overnight?

Most likely the mainspring wasn't close to full. A few hours of evening wear adds little charge, so give it a few dozen turns of the crown before wearing it and it will last about 41 hours.

Does the Seiko Cocktail Time Open Heart have a date?

No. The 4R38 shows only hours, minutes and seconds. Seiko removed the date so the balance could be seen through the dial at 9 o'clock.

Is the Seiko 4R38 the same as the NH38?

Yes, broadly. The NH38 is the version Seiko Instruments sells to other watch brands, with the same open-heart layout, 24 jewels and 41-hour reserve.

Is the 4R38 a good movement?

It's a reliable, easily serviced entry-level automatic based on the 4R36. Its weak points are the loose official accuracy spec and a reserve that won't cover a full weekend off the wrist.

Sources

• Seiko: Parts list and technical guide, Cal. 4R38A/4R39A (PDF)
• Seiko: Instruction manual for 4R35/4R36/4R37/4R38/4R39/4R71/4R72 (PDF)
• Seiko: Presage SSA441J1 product page
• Teddy Baldassarre: Seiko Presage Cocktail Time guide
• Grail Watch Reference: Seiko 4R38

Keeping it running

• On an average day an automatic gains roughly 4 hours of reserve per hour on the wrist, so about 10 hours of wear fills the Seiko 4R38 open heart. Desk days charge more slowly.
• Most automatics can also be wound by the crown. If yours has stopped, a few dozen turns gets it going before you put it on.
• Off the wrist it stops about 41 hours after it was last full. Stopped sooner?
• On a watch winder, start at about 650 turns per day, both directions. Winder settings for every movement.
• If it does stop, don't set the date between 9pm and 3am. Check if it's safe.

All Seiko movements

Never let it stop

Watch Winder counts down the reserve of every watch you own and reminds you before one stops. It already knows the Seiko 4R38 open heart: type your watch's name and the reserve fills itself in.

Manufacturer's published figure. Real reserve varies with the watch's age and service; check your manual.