Drop-in resolution
Seiko NH35 (TMI NH35A) vs Miyota 8218: do they swap?
Whether the Seiko NH35 (TMI NH35A) and Miyota 8218 share a case, and what changes if you move between them.
Different footprints. The Seiko NH35 (TMI NH35A) is 27.40 mm x 5.32 mm and the Miyota 8218 is 26.00 mm x 5.67 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Seiko NH35 (TMI NH35A) is the default modding and microbrand automatic, hacking and hand-winding at low cost. The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday.
They differ in diameter (27.40 mm vs 26.00 mm), height (5.32 mm vs 5.67 mm), calendar layout (centre seconds, date at 3 vs centre seconds, day and date at 3), winding behaviour. Larger than the Swiss core at 27.40 mm, so it will not drop into a case cut for a 25.60 mm ETA. It is the unbranded version of the Seiko 4R35 and shares that caliber's footprint and stem.
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.