Drop-in resolution
Sellita SW220-1 vs Miyota 821A: do they swap?
Whether the Sellita SW220-1 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Sellita SW220-1 (25.60 mm x 5.05 mm) and Miyota 821A (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 Sellita SW220-1 is the Sellita day-date, the drop-in alternative to the ETA 2836-2. The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism.
They differ in diameter (25.60 mm vs 26.00 mm), height (5.05 mm vs 5.67 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. The day-date member of the SW200 family, dimensionally matched to the ETA 2836-2 including the 5.05 mm height and day-and-date-at-3 layout.
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.