Seiko made most of its own movements, and the caliber number tells you more about a used watch than almost anything else in the listing. On a vintage Seiko the caliber is the first four digits of the reference, so a 6309-7290 has a 6309 inside. Our guide to Seiko reference numbers covers the rest of the code. This guide covers what each caliber in our catalog does, what it can't do, and what tends to wear out.
Three terms to know first
- Beat rate. This is how fast the balance swings, in beats per hour. Most Seikos here run at 21,600, which is six beats a second, so the seconds hand moves in small steps. A higher rate makes the sweep smoother.
- Hand-winding. On a watch that hand-winds, turning the crown winds the mainspring. Many Seiko automatics don't, and you start them by moving the watch. Monochrome found about thirty seconds of shaking was enough to get a stopped Seiko 5 going.
- Hacking. A hacking watch stops its seconds hand when you pull the crown out to set the time, so you can set it to the second. Seiko describes this on the 4R35 as a second-hand stop at the crown's second position.
Most of these automatics wind through the Magic Lever, a simple and efficient winding system Seiko introduced in 1959. It winds whichever way the rotor turns.
Vintage diver calibers
6105. The 6105 divers replaced the 62MAS in 1968 and ran until 1977. The asymmetric 6105-8110 arrived in 1972 and later picked up the Captain Willard nickname from Martin Sheen's character in Apocalypse Now. Fratello lists the movement at 17 jewels and 21,600 beats per hour, with a quickset date and no hand-winding.
6309. The Turtle movement ran from 1976 to 1988. It's a 17-jewel automatic that neither hand-winds nor hacks. You'll find it in the original Turtle, the 6309-7040, and in the slimmer 6309-7290, whose case shape carried on into the SKX.
7002. In 1988 Seiko replaced the 6309 with the 7002, which ran until 1996. It uses more plastic parts, it drops the day display, and it still can't be hand-wound. The date quicksets at the crown's first stop. The 7002-700A family led directly into the 7S26 SKX divers of 1996, and some Japan-market 7002 dials print the jewel count.
7548. This quartz caliber arrived in 1978 in the 150 metre 7548-700B family. Its case is the one the SKX later used, and the dial says QUARTZ. That same year Seiko also put quartz into its Tuna.
Modern workhorses
7S26. First sold in 1996, the 7S26 powers the SKX007, SKX009, SKX013, SKX173, the Orange Monster and the SNK809. It has 21 jewels, beats at 21,600, holds about 41 hours of power and shows day and date. It doesn't hack or hand-wind. Sellers sometimes misprint it as 7526A, so search both.
4R35 and 4R36. Introduced around 2011, these keep the same beat rate and add hacking and hand-winding. They have 24 jewels, though some 4R35B movements show 23, and they run for about 40 hours. The 4R35 shows only the date, as in the Cocktail Time SRPB43. The 4R36 adds the day and powers the SRPD55 and the SRP777 Turtle reissue. Seiko Instruments sells the same designs to other brands as the NH35 and NH36, so spare movements are easy to find.
6R15. This is the step up, found in the SARB033 and SARB035. It hacks, hand-winds and holds about 50 hours of power. The first Cocktail Time watches of 2010 used it too. Seiko revised it several times, and Caliber Corner notes that the early A, B and some C versions had problems Seiko fixed later. Parts don't always swap between versions, so a watchmaker will want to know which one you have.
Chronographs
6139 and 6138. These are Seiko's first automatic chronographs, built with a column wheel and a vertical clutch. The 1969 6139 in the 6139-6002 has one 30-minute register and no hand-winding. The 1970 6138 adds an hour register and hand-winding, as in the 6138-0040 Bullhead and the John Player Special. Both run at 21,600. Our guide to the 6139 and 6138 covers them in depth.
7A28. This quartz chronograph shows up in Seiko catalogs from 1983, and Seiko lists its analog quartz chronograph that year among its firsts. The 7A28 in the 7A28-7000 has 15 jewels and an all-metal build with no plastic parts. A watchmaker can service it, and its rate can be adjusted. A third pusher at 10 o'clock handles split times.
1960s specialists
5740C. The Lord Marvel of 1967 beats at 36,000 per hour, ten beats a second, which gives a very smooth sweep. Beyond the Dial calls it only the second watch in the world to run that fast, after a Girard-Perregaux the year before. It's hand-wound only, with no date, and it stayed in production unchanged until 1975.
4006. The Bell-Matic, made from 1966 to 1978, is a mechanical alarm watch with two mainsprings. The rotor winds the watch, and the crown winds only the alarm, which rings for about ten seconds. It runs at 19,800, doesn't hack, and came with 27, 21 or 17 jewels depending on year and market. The button at 2 o'clock arms the alarm and also quicksets the date.
6119. The everyday Seiko 5 day-date caliber from about 1967 into the mid 1970s, found in the Seiko 5 6119 range. It has 21 jewels, beats at 21,600 and runs about 40 hours. It doesn't hack or hand-wind. Pushing the crown in advances the date on the A and B versions, and the 6119C adds a bilingual day wheel. NASA flight director Gene Kranz wore a Seiko 5 6119-8460 during Apollo missions.
Which ones hack and hand-wind
- Both. 4R35, 4R36, 6R15.
- Hand-winds, chronograph. 6138.
- Hand-wound only. 5740C.
- Neither. 7S26, 6309, 6119, and the 4006, whose crown winds only the alarm.
- No hand-winding. 6105, 7002, 6139.
- Quartz. 7548 and 7A28.
Servicing realities
A modern 7S26 or 4R36 is simple to service, and replacement movements are easy to get. Vintage calibers are harder. On the 6139, the barrel arbor slowly wears its holes oval, and Ashton Tracy calls this a common wear point that needs fixing more often than not. Its chronograph clutch wheels eventually slip too, and good spares are getting hard to find. Any vintage case that's been opened over the years may also have lost its seals and pusher springs, so treat old water resistance ratings as history. Ask the seller when the watch was last serviced and what was done. Budget for a service on any vintage piece that has no paperwork.
