Seiko SPB317J1: specifications and what to check

A hundred grams, and most of the reason is the band

Seiko publishes the complete weight of this reference as 100.0 g.

For a steel diver rated to two hundred metres, that is very light — the 1965 Heritage on a bracelet is 168 g, and several of the steel chronographs documented on this site are heavier still.

The explanation is the silicone band. Swap it for a steel bracelet and a large part of that advantage disappears.

That is worth saying to a buyer who is choosing this reference because it wears lightly, and it is worth remembering yourself when someone asks whether a bracelet would suit it. The answer is yes, and it will no longer be a hundred-gram watch.

Silicone fails from the inside of a bend

The band is the part of this watch with the shortest working life, and it is almost never inspected properly.

Silicone and rubber do not wear like leather. They do not thin, fray or stretch in a way you can see lying flat on a bench. They perish, and perishing starts inside a crease — where the material folds around the spring bar and where it bends over the wrist.

So do not look at it flat. Take each end, bend it hard back on itself, and open the fold toward the light. Fine parallel cracks in the crease mean the band is at the end of its life whatever the outside looks like.

Two more checks while you are there. The spring bar holes should be round, not elongated. And the band should return to shape when you let go rather than holding a set.

A perished band on a diving watch is a safety matter, not a cosmetic one. If it has gone, price a replacement in and say so — it is a cheap part and an honest disclosure.

6R35, not 6R55

Two very similar calibres appear across the Prospex references documented here, and they are easy to confuse.

• 6R35 — this watch. Approximately 70 hours. 24 jewels. Date.
• 6R55 — the 1965 Heritage. Approximately 72 hours. 24 jewels. No date.

Both are stated at +25 to −15 seconds per day, both are automatic with manual winding, both are 24-jewel movements.

The date is the quickest tell in a photograph. The reserve is the tell in a specification. Quote the one that belongs to the reference in front of you rather than the one you remember, and check the date actually changes cleanly across midnight while you have the watch.

The usual diver's checks, briefly

Unidirectional bezel — it must refuse to turn clockwise, and its anticlockwise clicks should all feel the same.

Screw-down crown — thread it and unthread it several times, feeling for the catch on the first quarter turn.

LumiBrite on hands, indexes and bezel — all three should glow the same colour and fade together.

Curved sapphire with anti-reflective coating on the inner surface — hold it at a shallow angle under a light and look for interior bloom, which cannot be polished out.

Published dimensions: 41.0 mm diameter, 12.3 mm thick, 46.9 mm lug to lug, 20 mm lug width. Magnetic resistance is given as 4,800 A/m.

And as with any used watch carrying a diver's rating: the number came from the factory, not from a test anyone has done on this example.