Drop-in resolution
ETA 2836-2 vs Miyota 821A: do they swap?
Whether the ETA 2836-2 and Miyota 821A share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA 2836-2 (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 ETA 2836-2 is the day-date sibling of the 2824-2, in dressier and tool watches that show the weekday. 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. Same 25.60 mm diameter as the 2824-2 but taller at 5.05 mm because of the added day disc. A 2824-2 will not fill a 2836 dial: the day window sits empty.
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.