Seiko Caliber 6L37

The Seiko caliber 6L37 is an automatic 3-hander watch movement with a date complication. The 6L37 is part of the 6L family of slimline movements. It is based on the Seiko 6L35 and retains the same thickness profile of about 3.7mm thin.

This new movement was introduced in July of 2023 and is found in the new Seiko Prospex 1965 Diver’s Re-creation Limited Edition timepiece. The watches are slated to start shipping in August 2023.

In Seiko’s own words:

“The watch is powered by the new Caliber 6L37, the first movement from the slimline 6L family made expressly for diver’s watches. It features the same accuracy and power reserve as the trusted 6L35 movement that is the mainstay of the 6L family; however, thanks to modifications in construction and material, its durability and shock resistance now meet the strict standards of a Seiko Prospex diver’s watch.” –Source

6L37 vs 6L37A:

6L37 is the number of the caliber, with the trailing letter designating the version or evolution of the movement. You may see the movement referenced as 6L37 but upon opening your watch, the rotor may say 6L37A. Don’t be alarmed, 6L37 and 6L37A are the same thing since A is the first version. At this time, there is only a 6L37A and not 6L37B, 6L37C, etc.

6L37 VS 6L35:

There is a similar caliber 6L35 which is actually the base caliber for the 6L37 – they share the same framework/architecture. It is not clear exactly what was changed between the two movements, however, Seiko has stated that the newer caliber was created specifically for diving watches and with increased durability, specifically with regards to the shock resistance. Both calibers feature the same Seiko Diashock anti-shock device.

From the brand’s computer generated stock images, the only difference appears to be the caliber number on the rotor. See side-by-side image below:

Editor’s rant: Since a new caliber introduction is part of the marketing hype for a new model launch, consumers should expect brands to be more transparent about what the differences are between the previous caliber and the new number. What justifies a new caliber number? What justifies buying the watch based on the new movement?

Accuracy:

Seiko claims that the cal. 6L37 is factory adjusted for an average accuracy in the range of -10 to +15 seconds per day, when worn in normal conditions and temperatures between 5 to 35 degrees C (about 41 – 95 F).

A note from Seiko on accuracy expectations:

“Due to the characteristics of mechanical watches, any actual daily rate may not fall within the range of time accuracy specified above dependent on the conditions of use, such as the length of time during which the watch is worn on the wrist, temperature, arm movement, and whether the mainspring is wound up fully or not, etc.” -6L37 Instruction Manual, last page, fine print

Recommended Service Intervals:

Seiko officially recommends for 6L37 powered watches to be overhauled every 2-3 years.

“Periodic inspection and adjustment by disassembly and cleaning (overhaul) is recommended approximately once every 2 to 3 years in order to maintain optimal performance of the watch for a long time.” -6L37 Instruction Manual, Inspection and adjustment by disassembly and cleaning (Overhaul), page 38

Replacement Prices:

At the time of this post, the calibre 6L37 is not sold individually, so the only way to get one is to buy the whole watch. Also at this time, there is not a non-Seiko branded version of this movement (such as from Seiko’s other divisions, TMI, etc).

Watches with this movement:

Currently, the only watch with the caliber 6L37 is the aforementioned 1965 Diver’s Re-creation. This model is a numbered limited edition with only 1,965 watches available in the world. The cover of the instruction manual for this model labels it as a “Diver’s Watch For Air Diving”.

TL;DR / FAQs:

Is the 6L37 in any other watches?
No.

Is there an SII/TMI version for microbrands?
No.

Is it manual winding and hacking?
Yes.

Can the mechanism be over-wound?
No.

When should I not set the date?
Between 9PM and 1AM according to Seiko but, but be safe and do make adjustments between 8PM-2AM.

Editor’s Update: Interestingly, the 6L35 instruction manual calls for avoiding changing the date between 8PM-2AM as I originally stated above. Strange that the 6L37 changes it to 9PM and 1AM – maybe it just means that Seiko find the 8PM-2AM warning to be too conservative and they are confident that 9PM-1AM is an acceptable safe zone. Either way, it’s better the be safe and avoid that span altogether when advancing the datewheel – bu t you can also follow the manufacturer’s recommendations.

How many turns of the crown for the mainspring to be fully wound?
This is still unconfirmed, although Seiko does recommend winding the crown (slowing) for 20 turns when starting the watch from a stopped state.

Additional Resources:

• Official 6L37 instruction manual (pdf)
• List of Seiko movements