Drop-in resolution
ETA 2824-2 vs Miyota 821A: do they swap?
Whether the ETA 2824-2 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA 2824-2 (25.60 mm x 4.60 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 ETA 2824-2 is the reference Swiss workhorse automatic, used across mid-range Swiss brands for decades. 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 (4.60 mm vs 5.67 mm), beat rate (28,800 A/h (4 Hz) vs 21,600 A/h (3 Hz)), winding behaviour. The dimensional benchmark for the whole 11 1/2 ligne core. Any donor claiming 2824 compatibility is measured against 25.60 mm x 4.60 mm with date at 3.
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.