Drop-in resolution
Seiko 7S26 vs Miyota 8218: do they swap?
Whether the Seiko 7S26 and Miyota 8218 share a case, and what changes if you move between them.
Different footprints. The Seiko 7S26 is 27.40 mm x 4.90 mm and the Miyota 8218 is 26.00 mm x 5.67 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Seiko 7S26 is the long-running Seiko workhorse behind the SKX and Seiko 5, day-date, non-hacking. The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday.
They differ in diameter (27.40 mm vs 26.00 mm), height (4.90 mm vs 5.67 mm), winding behaviour. 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.
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.