The Seiko 6309 powered the original Turtle divers, the chunky cushion-case watches that modern Seiko Prospex Turtles copy. It is one of the most collected vintage Seiko calibers, which also makes it one of the most faked and frankened. This page gathers the facts you need before buying one.
Quick facts
| Spec | Detail |
|---|---|
| Caliber | 6309, automatic |
| Made | 1976 to 1988 |
| Jewels | 17 |
| Beat rate | 21,600 vph |
| Power reserve | 47 hours |
| Hacking / hand-winding | No / No |
| Display | Day and date, quickset |
| Lug width | 22 mm |
| Replaced by | Caliber 7002, from 1988 |
Known case references
| Reference | Nickname | Production window |
|---|---|---|
| 6309-7040 | Turtle | 1976 to 1988 |
| 6309-7290 | Slim Turtle, black | April 1982 to January 1988 (observed serials) |
| 6309-729A | Slim Turtle, Pepsi | May 1982 to January 1988 (observed serials) |
| 6309-729B | Slim Turtle, orange | May 1982 to April 1985 (observed serials, smaller run) |
The Slim Turtle windows come from serial numbers The Seiko Guy has recorded on surviving watches, so a date a few months outside them is a question, not proof of a fake.
What to check before you buy
- Serial date. The serial must decode to a date between 1976 and 1988, and inside the window of its case reference. A 6309-729B dated 1987 deserves questions.
- Movement. Open the back or ask for a movement photo. It should say 6309. A 6319 or an NH35 behind a 6309 caseback is a franken.
- Dial code. The tiny print at 6 o’clock should start with 6309. Reprinted dials are common, so look for crisp print and lume that is not glossy and painted on.
- Bezel and hands. Aftermarket inserts and hands are cheap and everywhere. Check that the lume on the hands has aged like the dial.
Need a strap? The 6309 takes 22 mm. Our Strap Size Finder suggests the best types for how you wear it. Want to compare it with modern Seiko calibers? See the movement comparison.
Looking for one?
Garasi Opung, the Jakarta dealer behind this site, hand-checks every vintage Seiko before listing it. Browse the Etsy shop, the eBay store or garasiopung.com, and run any listing’s caseback through the Franken Checker before you buy, ours included.
Frequently asked questions
Was the Seiko 6309 hand-windable?
No. The 6309 is automatic only and does not hack. If it has stopped, wear it or give it a gentle swirl to get it going.
What is the difference between the 6309-7040 and the Slim Turtle?
The 6309-7040 is the original full-size cushion-case Turtle. The Slim Turtle cases (6309-7290, 729A and 729B) arrived in 1982 and use the same movement in a slimmer case, in black, Pepsi and orange.
What lug width does a Seiko 6309 have?
22 mm, according to the StrapsCo lug width chart.