Seiko 7S26 caliber: dimensions and drop-in group

Used in: the long-running Seiko workhorse behind the SKX and Seiko 5, day-date, non-hacking.

The predecessor of the NH36 at 27.40 mm x 4.90 mm, without hand-winding or hacking. It shares the 27.40 mm case fit of the NH3x family, so an NH36 is the standard upgrade donor into a 7S26 case.

Drop-in group: Seiko 7S26 day-date footprint (27.40 mm x 4.90 mm). Matching diameter and height is necessary but not sufficient. A donor is only a true drop-in when the dial-foot positions, hand-hole sizes, stem length and calendar layout also match. Two movements can share a case and still need a new dial, re-broached hands or a cut-to-length stem.