Drop-in resolution
Sellita SW300-1 vs Miyota 821A: do they swap?
Whether the Sellita SW300-1 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Sellita SW300-1 (25.60 mm x 3.60 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 SW300-1 is the Sellita slim automatic, the drop-in alternative to the ETA 2892-A2. 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 (3.60 mm vs 5.67 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), winding behaviour. Dimensionally matched to the 2892-A2 at 25.60 mm x 3.60 mm, with four more jewels (25 vs 21). A current-production barrel lifts the reserve to 56 h without changing the caliber name or footprint.
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.