MovementSwap

Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko 6R15 is 27.40 mm x 5.25 mm, so they do not share a case. This is a choice between platforms, not a swap. Published dimensions, layout and the checks before you swap one for the other.

Updated 05 September 2026.

Drop-in resolution

Miyota 821A vs Seiko 6R15: do they swap?

Whether the Miyota 821A and Seiko 6R15 share a case, and what changes if you move between them.

No drop-in

Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko 6R15 is 27.40 mm x 5.25 mm, so they do not share a case. This is a choice between platforms, not a swap.

The Miyota 821A is the decorated 8215, with Cotes de Geneve and perlage on the same mechanism. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.

They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.25 mm), winding behaviour. Mechanically and dimensionally the 8215 at 26.00 mm x 5.67 mm, dressed up with finishing. It is a genuine drop-in for an 8215 case: the footprint, stem and dial feet are identical.

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.

Calibers and lookups

Miyota 821A26.00 mm x 5.67 mm
Seiko 6R1527.40 mm x 5.25 mm
Donor movement for Miyota 821Adrop-in lookup
Donor movement for Seiko 6R15drop-in lookup

Questions and answers

Miyota 821A vs Seiko 6R15: do they swap?

Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko 6R15 is 27.40 mm x 5.25 mm, so they do not share a case. This is a choice between platforms, not a swap.