Looking into a Seiko Watch’s Movements – Part 2

Seiko has created excellent watches for centuries, launching itself to the top of the watchmaking industry. With a wide range of well-loved timepieces, it is truly a master in its craft, producing one horological work of art after another. Its watch movements have also propelled its popularity, revolutionizing quartz watches with its own Seiko Quartz Astron.

In our previous article, we discussed the basics of watch movements and what Seiko offered to consumers. This article will further explore how Seiko set itself apart from its competitors through its watch movements. Here’s what you need to know:

The Popularity of the 7S Base

Even though it was an innovator in quartz watch movements, Seiko never neglected the mechanical counterparts. The 7S line is responsible for powering the famous SKX divers, easily one of Seiko’s most popular watches. The 7S line of movements includes the 7S25, 7S26, 7S35, and 7S55, used in many Seiko models. They all lack hacking and hand-winding but make use of a traditional mainspring.

Seiko introduced the 7S26 in 1996, equipping it with a 40-hour power reserve and a 21-jewel movement with daily timekeeping specs of -20 to +49 seconds. Admittedly, it isn’t as accurate as the other movements in the 7S series, but it proved popular all the same. Seiko added jewels to minimize friction between moving parts, boost accuracy, and extend the bearings’ life for better performance. However, the gems are usually synthetic sapphire or ruby. 

The 7S26 penetrated the market through the inside of the SKX007 and the resulting SKX line, particularly when the models were introduced in 1996. The SKX’s success propelled the 7S26 to the popularity it enjoys today as one of the most loved automatic movements.

The 6R15: A Leap of Improvement

The 6R series of movements is in Seiko models like the Alpinist, Sumo, SPBXXX diver, and the trendy SARB line. The 6R15 has many similarities with the 7S series. It is a date-only movement since it doesn’t have a day indicator, but it provides significant upgrades from the 7S series. For instance, the 6R has both handwinds and hacks, modern features the 7S series doesn’t have. It also has the Spron 510 mainspring, also made by Seiko, pushing the power reserve from 40 hours to 50 hours. 

The 6R vibrates at 21,600 vph like the 7S series, although it contains 23 jewels instead of the 21 that the 7S26 has. It also boasts a rated accuracy of +25/-15 seconds each day, but its owners observe a higher accuracy. Recently, the company introduced the 6R35, which increases the power reserve up to 70 hours and brings up the jewel count to 24. It is more often used in the more expensive models of Seiko’s midrange watches. 

The NH35 and NH36, Popular Movements for Seiko Mods

The Seiko modding community is particularly fond of the NH35A and NH36A, unbranded versions of the 4R35 and 4R36 movements, all manufactured by Seiko. The 4R35 and 4R36 are hybrids between the 7S and 6R series, adding hacking and hand-winding. However, the NH35A and NH36A are often in microbrand watches.

The NH35 has a day complication, while the NH36 has both the day and date, making it the ideal upgrade. Since it is based on the 7S line, all original SKX and aftermarket dials can be used in tandem with the NH36 without extra modification. The NHs boast better-operating specs than the 7S line, offering 21,600 vph and an accuracy of +45/-35 seconds per day. Since Seiko mod parts are designed to fit SKX cases and movements, the NH line is the optimal choice of movement due to its modern features and backward compatibility.


Seiko is renowned for its stunning watches and the mechanisms inside, particularly the watch movements. Now that you understand watch movements better, you’ll know the right watch mod parts to go with your Seiko timepiece!

If you’re looking for an NH36 movement for your watch, check us out at SeikoMods! We are the leading provider of Seiko mod parts, such as custom Seiko dials, SKX bezel inserts, chapter rings, and more. Contact us today to find out more about our mod parts!

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