The Seiko caliber 7S26 is an automatic movement used in many popular Seiko divers and the entry level Seiko 5 watches. This movement was first released to the market in 1996. The most recent tech sheets found were dated 2011.
Modern Seiko watches, such as the Next Gen Orange Monster, have since been upgraded to the caliber 4R36, making Seiko collectors happy because the 7S26 seen here does not hack and is not hand-windable – two functions that have become more and more popular.
7S26A VS 7S26B VS 7S26C
The letter indicates the evolution of the caliber 7S26. 7S26 and 7S26A is the first generation of this caliber and are the same. 7S26B is the second generation. 7S26C is the third and final version.
The difference between A/B in Seiko’s own words:
“Cal. 7S watches are changed from caliber “A” to caliber “B” in October 2006 production. According to the change, we would like you to pay attention to the design of the balance staff when repairing those watches.” -Source: Seiko Parts Catalogue/Technical Guide Cal. 7S26B, 7S36B. Published 2007.
The difference between B/C in Seiko’s own words:
“SEIKO Automatic Mechanical Cal. 7S26C / 7S36C are replacement caliber of Cal. 7S26B / 7S36B. Construction of the C series is same as B series, but using new parts. Since the size of movement is same as B series, the complete movement can be assembled into the watches which originally have the B series move-ment; however, as the parts are not convertible, please use the appropriate parts for each caliber.” -Source: Seiko Parts List/Technical Guide Cal. 7S26C, 7S36C. Published 20011.
7S26 VS 7S36
The main difference between the 7S26 and the 7S36 is that the caliber 7S36 has 2 extra jewels. The 7S26 has 21 jewels and the 7S36 has 23 jewels. Many enthusiasts refer to this as 2 non working jewels. There is speculation that the 7S36 was supposed to eventually replace the 7S26, but that never actually happened.
The extra 2 jewels are on the “lower bridge for 3rd wheel and pinion”. They are lower cap jewels for the 3rd and escape wheels. But that’s a bit of a fancy answer – here’s an easy one: they are basically useless jewels and do nothing to alter performance… To answer your question, as far as complete swapouts go, they are interchangeable. But that would be a practice in futility – you wouldn’t gain or lose anything but (your own) time. I have both of these movements, have for years, and there is no practical difference. But to repair, there are some things to consider. Some of the parts are different in size – you can find the details around here somewhere, but the 3rd and escape wheels have different axle lengths due to the jeweled capping. Not to mention this bridge that simply does not exist in the 7s26. Maybe something else equally minor.