Drop-in resolution
Miyota 8218 vs Miyota 9012: do they swap?
Whether the Miyota 8218 and Miyota 9012 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Miyota 8218 (26.00 mm x 5.67 mm) and Miyota 9012 (26.00 mm x 3.90 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 8218 is a day-date 8200 member, in entry automatics that show the weekday. The Miyota 9012 is a 90-series premium automatic with a 24-hour sub-hand and date.
They differ in height (5.67 mm vs 3.90 mm), beat rate (21,600 A/h (3 Hz) vs 28,800 A/h (4 Hz)), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3, 24h sub-hand), 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.