MovementSwap

Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko NH34 (TMI NH34A) is 27.40 mm x 5.32 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 NH34 (TMI NH34A): do they swap?

Whether the Miyota 821A and Seiko NH34 (TMI NH34A) 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 NH34 (TMI NH34A) is 27.40 mm x 5.32 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 NH34 (TMI NH34A) is the affordable modding GMT automatic, adding a caller-GMT hand to the NH35 base.

They differ in diameter (26.00 mm vs 27.40 mm), height (5.67 mm vs 5.32 mm), calendar layout (centre seconds, date at 3 vs centre seconds, GMT 24h hand, date at 3), 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 NH34 (TMI NH34A)27.40 mm x 5.32 mm
Donor movement for Miyota 821Adrop-in lookup
Donor movement for Seiko NH34 (TMI NH34A)drop-in lookup

Questions and answers

Miyota 821A vs Seiko NH34 (TMI NH34A): do they swap?

Different footprints. The Miyota 821A is 26.00 mm x 5.67 mm and the Seiko NH34 (TMI NH34A) is 27.40 mm x 5.32 mm, so they do not share a case. This is a choice between platforms, not a swap.