MovementSwap

Different footprints. The Miyota 8218 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 8218 vs Seiko NH34 (TMI NH34A): do they swap?

Whether the Miyota 8218 and Seiko NH34 (TMI NH34A) share a case, and what changes if you move between them.

No drop-in

Different footprints. The Miyota 8218 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 8218 is a day-date 8200 member, in entry automatics that show the weekday. 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, day and date at 3 vs centre seconds, GMT 24h hand, date at 3), winding behaviour. A day-date 8200-family caliber at 26.00 mm x 5.67 mm, sharing the 8205's day-and-date layout and the family footprint. It drops into an 8200-series case with a matching day-date dial.

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 821826.00 mm x 5.67 mm
Seiko NH34 (TMI NH34A)27.40 mm x 5.32 mm
Donor movement for Miyota 8218drop-in lookup
Donor movement for Seiko NH34 (TMI NH34A)drop-in lookup

Questions and answers

Miyota 8218 vs Seiko NH34 (TMI NH34A): do they swap?

Different footprints. The Miyota 8218 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.