Drop-in resolution
Miyota 8215 vs Miyota 821A: do they swap?
Whether the Miyota 8215 and Miyota 821A share a case, and what changes if you move between them.
Interchangeable. The Miyota 8215 and Miyota 821A share the same 26.00 mm x 5.67 mm footprint and the same centre seconds, date at 3 layout, so one drops into the other's case with the correct dial and hands.
The Miyota 8215 is the low-cost Japanese workhorse, common in entry-level and fashion automatics. The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism.
Their published figures are the same to the tenth of a millimetre. Close to the Swiss 11 1/2 core in diameter (26.00 mm) but taller at 5.67 mm and with a different dial-foot pattern, so it is not a drop-in for an ETA 2824 dial. Its seconds hand also has a soft, non-hacking stutter that is characteristic of the caliber.
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.