Drop-in resolution
Miyota 90S5 vs Seiko NH38 (TMI NH38A): do they swap?
Whether the Miyota 90S5 and Seiko NH38 (TMI NH38A) share a case, and what changes if you move between them.
Different footprints. The Miyota 90S5 is 26.00 mm x 3.90 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 90S5 is the open-heart member of the 90 series, showing the balance through the dial. 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 (3.90 mm vs 5.32 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (open heart, no date vs centre seconds, open heart, no date), winding behaviour. A 90 series caliber at 26.00 mm x 3.90 mm with an open-heart aperture that exposes the balance. It shares the 9015 footprint and dial feet but needs a dial cut for the open-heart window.
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.