Drop-in resolution
Miyota 9039 vs Miyota 8218: do they swap?
Whether the Miyota 9039 and Miyota 8218 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Miyota 9039 (26.00 mm x 3.90 mm) and Miyota 8218 (26.00 mm x 5.67 mm) are near in size but differ in height, dial feet or layout, so moving between them means a new dial, hands and often a case adjustment.
The Miyota 9039 is the no-date member of the 90 series, popular in clean-dial microbrand watches. The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday.
They differ in height (3.90 mm vs 5.67 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (centre seconds, no date vs centre seconds, day and date at 3), winding behaviour. The no-date sibling of the 9015 at 26.00 mm x 3.90 mm, sharing the same dial-foot location. It drops into a 9015 case as long as the dial has no date window, which is its whole point.
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.