Drop-in resolution
Miyota 821A vs Miyota 9039: do they swap?
Whether the Miyota 821A and Miyota 9039 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Miyota 821A (26.00 mm x 5.67 mm) and Miyota 9039 (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 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism. The Miyota 9039 is the no-date member of the 90 series, popular in clean-dial microbrand watches.
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, date at 3 vs centre seconds, no date), winding behaviour. Mechanically and dimensionally the 8215 at 26.00 mm x 5.67 mm, dressed up with finishing. It is a genuine drop-in for an 8215 case: the footprint, stem and dial feet are identical.
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.