Drop-in resolution
Miyota 8218 vs Seiko NH38 (TMI NH38A): do they swap?
Whether the Miyota 8218 and Seiko NH38 (TMI NH38A) share a case, and what changes if you move between them.
Different footprints. The Miyota 8218 is 26.00 mm x 5.67 mm and the Seiko NH38 (TMI NH38A) is 27.40 mm x 5.32 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday. The Seiko NH38 (TMI NH38A) is the open-heart, no-date member of the NH3x family, in skeleton-dial mod watches.
They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 mm), calendar layout (centre seconds, day and date at 3 vs centre seconds, open heart, no date), winding behaviour. A day-date 8200-family caliber at 26.00 mm x 5.67 mm, sharing the 8205's day-and-date layout and the family footprint. It drops into an 8200-series case with a matching day-date dial.
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.