Seiko 6309-7040

This is the volume Seiko diver of the late seventies and eighties — the watch that put a 150 metre automatic on ordinary wrists. It was released in 1976, and Watches of Espionage gives its US retail price in autumn 1977 as $155. It sits in a straight line: the 62MAS of 1965, the 6105 of 1968 and 1970, the 6306 and 6309 from 1976, the 7002 from 1988, then the SKX. The cushion case, the recessed crown at four o'clock and the bezel with two rows of grip teeth are what people mean by "Turtle", and Seiko brought the design back in its Prospex range in late 2015. On the reference: the 7040 and the 7049 are the same watch mechanically and dimensionally — a side-by-side comparison on WatchSleuth returns identical figures for everything except the case code. The usual reading is that the 7049 went to North America and the 7040 to other export markets; Japan had the 6306. That is the majority position among the sources we could check, but it does not rest on a Seiko document we were able to open, so treat it as the likely story rather than a fact. The claim that 7040 is early and 7049 is late is not supported by anything we could verify. Keep it apart from the 6306, which is the Japan-only sister with 21 jewels and hacking seconds, and from the 6309-729x "Slim Turtle", which is a different and smaller case. On military use, which is where the loose talk starts: Watches of Espionage says they were commonly bought by service members at the PX or Navy Exchange, and publishes photographs of US Navy divers and others wearing them. The best-documented individual example we found is a 6309-7049 issued to Master Chief Rudy Boesch in 1979. But that provenance rests on Boesch's own account, a 2013 photograph and a 1996 charity auction, not on markings on the watch. Any individual example should be described as the type widely used by military divers, never as military issue, unless it comes with its own documented provenance.

A genuinely wearable vintage tool watch. Legible, tough, and 45 millimetres across but short lug to lug, so it wears smaller than the number suggests. Parts and donor movements are still plentiful, and it is cheap enough that you actually wear it rather than keep it in a drawer. Swimming and showering are realistic, but only after a fresh gasket set and a pressure test. Where it falls short: the crown neither winds the movement nor stops the seconds hand. You cannot set it to the second, and you must shake or wear it to start it. The bezel turns both ways, so a knock can move it and make it show less elapsed time than has passed. Do not rely on it as a dive instrument. Hardlex is mineral glass, not sapphire, and the crystal is flat, so scratches show. Accuracy: we could not find a Seiko tolerance figure for the 6309. Expect tens of seconds a day, not seconds. It was never a chronometer and a watch running a minute fast is a service candidate, not a broken watch. And originality is the value driver, while many survivors have been apart at least once.

Not necessarily broken. A rate that swings from one position to another, or between full wind and nearly run down, is what a simple automatic can do when its lubricant has aged. Wear it a day and check it over twenty-four hours. If you want a number rather than an impression, a timegrapher reading showing rate, beat error and amplitude will tell you in a minute what a week of wearing tells you vaguely. Have it serviced before judging it.

The most important physical check on this watch, and one of the expensive things that can be wrong with it. Screw and unscrew it by hand. It should catch cleanly within a fraction of a turn and tighten progressively. Warning signs: it spins without biting, it cross-threads, it needs to be pressed hard inward before it catches, it feels gritty, or the crown wobbles when unscrewed. This matters more on a 6309 than on most watches because the crown tube is machined as part of the case, not screwed in. Strip the threads and you are looking at case work, not a spare part.

Advance the date with the quickset while you watch, and cycle the hands through midnight. A date that jumps late, sticks mid-change, needs a nudge, or will not quickset at all can indicate damaged calendar parts — a real fault, and a different one from the lubrication story above.

Hold it to your ear and rock it gently. A healthy rotor whirrs. A worn one rattles, grinds, or can be felt scraping. A whirr is the watch working.

Before anything dramatic: an automatic that has sat in a drawer has simply run down. The 6309 cannot be wound from the crown, so wear it, or give it a gentle shake or a slow rotation in the hand to start it, before concluding anything.

The case back notches for tool marks and gouges, which mean repeated amateur opening. Then look through the glass for fogging or water marks on its underside, discoloured patches at the outer edge of the dial, and rust flecks where the dial meets the case. Water in a watch of this age is usually a fault of the gaskets, and you should assume they are original unless someone proves otherwise.

Originality, which is a different question from condition. On an original dial the six o'clock marker sits clearly above the small print at the bottom edge; Worn & Wound notes that on common fake dials it runs down into it. The day and date aperture on an original is bevelled — deeply on the early Suwa dials, less so on later ones — and fakes often have no bevel at all. Dial and hands should have aged together, with matching lume colour — mismatched brightness between them, or a vivid bezel insert against an aged dial, suggests parts have been swapped. And the serial encodes the year and month of case production, so it should sit inside the production run and match the dial style.

The 6309 is a plain 17-jewel automatic with nothing beyond a calendar, which makes it one of the cheaper and simpler vintage movements to have serviced. Parts and donor movements are plentiful, though the rotor frame assembly is listed as obsolete. It is not on Seiko's list of calibres it refuses to service, but it is not on the price list either, so in practice an independent watchmaker does the work. Seiko suggests a service about every three years for its mechanical watches. Unless there is a dated receipt, assume none has been done. The job is the usual one: the movement taken apart, cleaned, oiled and regulated, then new gaskets and a pressure test. Budget for a gasket set and a crystal on top. On this watch the crown and its gasket matter more than the movement. The tube is part of the case, so have the watchmaker check the crown threads and seal before trusting any water resistance — see the crown tube and water resistance sections below.

The crown tube is an integral part of the case, not a screw-in part. The threads on the tube, or the female threads in the crown, strip from repeated screwing and unscrewing, and once they do the tube cannot simply be swapped. One owner reports the crown separating while diving once the threads were compromised. In order of preference: fit a different crown if the tube threads are sound; have a watchmaker drill out the damaged insert and fit an aftermarket tube assembly, a repair one restorer reports, on a related Seiko diver, still watertight after seven years; or transplant everything into a donor case. Owners report that a professional tube replacement often costs more than a replacement case. Not a home job.

The crystal, case back and crown are all sealed by gaskets, and on a watch this age assume they are original unless proven otherwise. Damaged crown threads undermine the crown seal. Gaskets are available. Fitting them and testing the result is a watchmaker's job: testing needs a pressure tester. The rating on the dial is a historical specification. Treat the watch as not water resistant unless it has been freshly gasketed and pressure-tested, and ask for the date of that test.

The insert fades, chips at the edges and loses its lume pip. A bezel that has stopped turning needs taking off and cleaning; do not force it. Replacement inserts are widely sold. But be honest about it: a replaced insert is legitimate and it stops the watch being all original. A fresh insert against an aged dial is the usual giveaway.

The quickset breaks when it is used while the movement's own midnight date-change train is already engaged. A rule widely given for quickset calendars is never to use the quickset between nine at night and four in the morning; during that window it interferes with the automatic advance and damages the gear responsible for it. The safe habit is to set the hands to roughly half past six in the morning first, so you know unambiguously where midnight is, and then quickset. The repair is replacement calendar parts from a donor — a watchmaker's job, but a movement-only one and not expensive next to a case repair.

Dried and thickened oil, dirt and magnetisation change the rate. Demagnetising is a two-minute job and should be tried first; beyond that it is a full service and regulation. Left alone, the old oil thickens and holds on to wear debris, so amplitude falls, the power reserve shortens, and the pivots and barrel arbor start to wear metal on metal. That is the point at which a modest service becomes a bigger one.

The bearing wears, the rotor droops and rubs — in one documented case against the edge of the spring barrel. The bearing is not separately replaceable; it is integral to the frame assembly, which suppliers list as obsolete. That means a used part from a vintage Seiko specialist, a donor movement, or a transplant. Check first that the rotor screw is not simply loose, which mimics the symptoms exactly.

Original examples in good original condition are getting hard to find, and aftermarket dials and hands are common. Nothing undoes a replaced dial; what a seller owes the buyer is a plain account of it. Collectors pay for originality, so a redial matters. For a watch being sold: photograph the dial at high magnification, the case back, the movement and the serial, and state plainly which parts are original and which are replacements.